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PLATE  I 


Frontispiece .] 


JULY  AT  HAMPTON  COURT. 
Reproduced  from  a Raydex  photograph. 


PHOTOGRAPHY  IN 
COLOURS 


BY 

GEORGE  LINDSAY  JOHNSON 

M.A.,  M.D.,  B.S.,  F.R.C.S. 

FELLOW  OF  THE  ROYAL  SOCIETY,  ITALY  (MODENA) 

FELLOW  OF  THE  ROYAL  PHOTOGRAPHIC  SOCIETY  ; LATE  EXAMINER 
IN  PHOTOGRAPHY  AND  THEORETICAL  AND  APPLIED  OPTICS  TO  THE  SPECTACLE 
MAKERS’  COMPANY,  LONDON  ; ETC.,  ETC. 


WITH  FOURTEEN  FULL-PAGE  PLATES  (FIVE  IN  COLOUR) 
AND  NUMEROUS  ILLUSTRATIONS  IN  THE  TEXT 


NEW  AND  REVISED  EDITION 


NEW  YORK 

E.  P.  DUTTON  AND  COMPANY 


1917 


Copyright  Registered 
All  Rights  of  Translation  reserved 


PRINTED  IN  GREAT  BRITAIN 


THE  GETTY  RESEARCH 
INSTITUTE  LIBRARY 


PREFACE 

TO  THE  THIRD  EDITION 


The  writer  has  not  found  it  necessary  to  make  many  altera- 
tions in  this  edition.  In  fact  he  has  only  corrected  a few 
errors  in  the  text,  and  rewritten  page  91  (§  48)  which 
answers  the  question  “ How  the  appearance  of  White  is  pro- 
duced on  a Colour  plate.”  He  regrets  that  he  did  not  give 
sufficient  credit  to  Professor  Forster  for  his  interesting  dis- 
covery of  the  reason  why  white  can  be  photographed  as 
white  on  a colour  plate,  and  he  has,  therefore,  rewritten  the 
paragraph  relating  to  it. 

But  if  few  alterations  have  been  found  necessary,  he  has 
been  obliged  to  enlarge  the  book  somewhat,  owing  to  the 
progress  which  has  been  made  since  the  last  edition  was 
written.  A full  description  of  the  Raydex  process  has  been 
added,  as  well  as  Gaumont’s  new  method  of  Cinematography 
in  Colours,  and  Carrara’s  method  of  reproducing  Auto- 
chromes on  paper. 

A chapter  has  been  added  on  Art  in  Colour  Photography, 
and  also  a further  chapter  on  Photomicrography  in  Colour. 
The  writer  desires  to  express  his  sincere  thanks  to  Mr. 
Walter  Severn,  A.R.C.Sc.,  F.C.S.,  for  revising  the  latter. 
His  great  experience  in  this  department  of  microphoto- 
graphy as  Assistant  Government  Bacteriologist  in  Cape  Town 
is  a guarantee  of  its  accuracy.  He  desires  to  add  that  the 
apparatus  arranged  for  taking  microphotographs  by  com- 
bining a vertical  microscope  with  a horizontal  camera  is 
entirely  Mr.  Severn’s  own  invention,  as  are  the  special 
methods  of  staining  the  slides. 

Castle  Mansions, 

Johannesburg,  1916. 


PREFACE 

TO  THE  FIRST  EDITION 


Since  the  publication  of  my  previous  work,  “ Photographic 
Optics  and  Colour  Photography,”  the  advances  made  in 
Photography  in  Colours  have  been  so  great  that,  rather 
than  revise  the  former  section  on  Colour,  it  was  thought 
desirable  to  write  an  entirely  new  work  on  the  subject, 
which  should  embody  all  the  latest  methods.  This  I have 
endeavoured  to  do,  and  it  is  hoped  that  the  Amateur  will 
find  all  the  information  he  requires  in  this  work  to  practise 
any  of  the  recognised  colour  methods  with  success.  There 
is  a general  impression  among  Amateurs  that  both  single  - 
plate  and  three -plate  colour  photography  present  great 
difficulties  to  the  beginner,  but  I can  assure  the  reader  that 
it  is  not  the  case.  Single-plate  colour  processes  at  any  rate 
are  quite  as  easy  to  manipulate  as  ordinary  dry-plates,  and 
occupy  even  less  time  than  the  latter. 

The  striking  analogy  which  exists  between  the  physio- 
logical perception  of  colours  and  the  phenomena  associated 
with  colour  photography  has  convinced  me  that  both  the 
ophthalmic  surgeon  and  the  physiologist  who  have  taken  up 
the  study  of  colour  blindness  and  colour  vision,  will  find  that 
the  serious  study  of  this  fascinating  science  will  illuminate 
many  obscure  phenomena  connected  with  the  physiology  of 
vision  and  colour  blindness,  and  will  well  repay  them  for 
the  time  spent  in  acquiring  a practical  knowledge  of  at  least 
one  of  the  leading  processes  described  herein. 

I beg  to  offer  my  thanks  to  those  who  have  assisted  me 
in  revising  the  work,  and  also  to  the  makers  of  the 
“ Autochrome,”  the  “ Dioptichrome,”  the  “ Omnicolore,” 
and  the  “ Thames  ” colour  plates,  for  revising  the  sections 
devoted  to  their  processes,  as  well  as  for  a large  number  of 
practical  hints  scattered  throughout  the  work. 

Lastly,  my  best  thanks  are  due  to  Dr.  Kenneth  Mees  for 
elucidating  several  points  of  difficulty,  as  well  as  to  the 
Editors  of  the  British  Journal  of  Photography  for  the  loan 
of  the  Photomicrographs  of  the  various  “ colour- screens.” 


G.  LINDSAY  JOHNSON. 


CONTENTS 

CHAPTER  I 

THE  NATURE  OF  LIGHT  AND  COLOUR 

PAGE 

§ 1.  Sources  of  Light 1 

§ 2.  Nature  of  Light 1 

§ 2a.  Ether 3 

§ 3.  Nature  of  Ether  Waves 4 

| 4.  Brief  Outline  of  the  Wave  Theory 5 

| 5.  Electro-magnetic  Theory  of  Light 7 

§ 6.  Nature  of  White  Light 9 

§ 7.  Cause  of  Sensation  of  Colour  11 

§ 8.  How  Colour  is  produced — Colour  of  Froth,  Powders,  and 
Ice — Cause  of  Whiteness  in  Clouds — Cirrhus  and  Rain 

Clouds 12 

§ 9.  Coefficient  of  Absorption  and  Transmission  of  Light — Lam- 
bert’s Law — Dichromatism — Colour  of  Pigments  . . 15 

§ 10.  Surface  Colours . .18 

§11.  Saturation  . 19 

§12.  Transparency  and  Translucency .20 

§ 13.  Reflection 21 

§ 14.  Shadows  ..........  22 

§ 15.  Coloured  Shadows  ........  25 

§ 16.  Shadows  cast  by  Lenses 25 

§ 17.  Colour  of  the  Sky — Tyndall’s  Experiments.  ...  26 


CHAPTER  II 

§ 18.  THE  EVOLUTION  OF  COLOUR  PHOTOGRAPHY 

oethe  — Zenl<  er  — W iener  — Lippmann  — Y oung  — Clerk 
Maxwell  — Helmholtz  — Ducos  du  Ilauron  — Ives  — 
Lumifere  Freres — Dr.  J.  H.  Smith- — Urban  and  Smith  . 29 


X 


CONTENTS 


CHAPTER  III 


THE  SENSATION  OF  COLOUR 


PAGE 

§ 19.  Description  of  the  Eye — The  Retina  and  Fundus  Macula — 
Fovea — The  Eye  compared  with  a Camera,  and  the  Retina 

with  a Colour  Plate 34 

§ 20.  Reason  why  the  Yellow  Spot  is  Yellow  ....  44 

§ 21.  Remarkable  Similarity  between  the  Autochrome  Colour 
Screen  and  the  Colour  Screen  in  certain  Birds  and 
Reptiles  ..........  45 

§ 22.  Colour  Vision  and  Colour  Blindness 47 

§ 22a.  The  Visual  Purple 53 

§ 23.  The  Meaning  of  the  Sensation  called  Black  ....  54 


CHAPTER  IV 


§ 24.  THE  SENSITIVENESS  OF  A PHOTOGRAPHIC  PLATE  AS  COMPARED 
WITH  THE  EYE  TO  DIFFERENT  PARTS  OF  THE  SPECTRUM 

Curves  of  Sensitivity — Panchromatic  and  Orthochromatic 

Plates 56 

§ 25.  Purkinje  Phenomenon 59 

CHAPTER  V 


METHODS  OF  OBTAINING  PHOTOGRAPHS  IN  COLOUR 

§ 26.  Lippmann’s  Interference  Method — Newton’s  Rings — Other 

Interference  Colours 63 

§ 27.  Theory  of  Colour  Formation — Making  a Three-Colour 

Transparency 67 


CHAPTER  VI 

SINGLE-PLATE  COLOUR  PROCESSES 


§ 28.  Joly’s  Ruled-Line  Screen  Process 71 

§ 29.  Comparison  between  the  various  Screen-plates — Table 

giving  characteristic  features  of  each  . . . .73 

§ 30.  Parallax 75 

§ 31.  Jougla’s  “ Omnicolore  ” Plate 76 

§ 32.  Dufay’s  “ Diopti chrome”  Plate  76 

§ 33.  The  Thames  Screen-plate 77 

§ 34.  Combined  and  Separate  Screen-plates  compared  . . 78 

§ 35.  Paget  Plate 79 

§ 36.  Development  of  Colour  Plates 81 

§ 37.  Combined  Paget  Plate 84 

§ 38.  Lumiere  “ Autochrome  ” Plate 84 

§ 39.  Relative  Speeds  of  Colour  Plates 85 


CONTENTS 


XI 


CHAPTER  YII 

SINGLE-PLATE  PROCESSES  DIAGRAMMATICALLY  EXPLAINED 

PAGE 

§ 40.  Yon  Hiibl’s  Diagram 86 

§ 41.  First  Black  Condition 89 

| 42.  Second  Black  Condition 90 

& 43.  How  the  Appearance  of  White  is  produced  on  a Colour 

Plate 90 


CHAPTER  VIII 

PRACTICAL  DETAILS  OF  THE  WORKING  OF  SINGLE  COLOUR-SCREEN 
PLATES 

§ 44.  Choice  of  a Plate 93 

§ 45.  Apparatus  and  Manipulation  required  for  making  a.  Single 

Colour-plate  Picture 95 

§ 45—1.  The  Camera 96 

§ 46 — 2.  The  Lens 97 

§ 47 — 3.  Choosing  the  Subject — Exposure 99 

§ 48 — 4.  Insertion  of  Plate  into  the  Slide 100 

| 49 — 5.  Colour  Filters 101 

§ 50—6.  Focussing 103 

§ 51—7.  Use  of  Hood 104 

§ 52 — 8.  The  Exposure 105 

§ 53 — 9.  Dark-room  Lamp  or  Safelight 106 

§ 54 — 10.  Formation  of  the  Coloured  Positive  ....  108 

§ 55 — 11.  First  Development 108 

§ 56.  Rules  for  Development 109 

§ 57 — 12.  Reversal  of  Image Ill 

§ 58 — 13.  Second  Development Ill 

§ 59 — 14.  Clearing 112 

§ 60 — 14a.  Hardening 112 

§ 61 — 15.  Intensification 113 

§ 62.  Explanation  of  the  Process  of  Intensification  . . .113 

§ 63 — 16.  Reduction 117 

§ 64 — 17.  Drying  the  Positive 118 

§ 65 — 18.  Varnishing 119 

§ 66 — 19.  Covering  the  Positive 119 

§ 67 — 20.  Final  Improvement  of  the  Tones  of  the  Image  . . 121 

§ 68 — 21.  Binding  up  the  Colour-screen  and  Transparency  . . 122 

§ 69 — 22.  Defects  in  Colour-Plate  Positives 123 

§ 70 — 23.  Copying  Colour  Plates 129 

§ 71 — 24.  Indoor  Portraiture 133 

§ 72 — 25.  Lantern  Projection  in  Natural  Colours  . . .134 

§ 73 — 26.  Resensitizing  Colour-screen  Plates  ....  135 

§ 74 — 27.  Preparing  Light  Filters 137 

§ 75.  Stereoscopic  Effect  of  Colour  Pictures  . . . .138 

§ 76.  Colour-screen  Filters  for  Monochromatic  Light  . . . 138 


Xll 


CONTENTS 


CHAPTER  IX 

THREE-PLATE  AND  TWO-PLATE  COLOUR  PHOTOGRAPHY 


PAGE 

§ 77.  Theory  of  Three-colour  Photography  ....  141 

§ 78.  Ives’  Kromskop 142 

§ 79.  Colour  Filters . 145 

§ 80.  Testing  of  Three-plate  Filters 148 

§ 81.  Making  Three-plate  Negatives 148 

§ 82.  Butler’s  Three-plate  Camera 149 

§ 83.  Two-plate  Colour  Photography 152 


CHAPTER  X 

THREE-PLATE  PHOTOGRAPHIC  COLOUR  PRINTING 

§ 84.  Colour  Prints 154 

§ 85.  Practical  Details  for  working  the  Three-Plate  Method 


with  Butler’s  Camera 154 

§ 86.  Three-colour  Half-tone  Process 158 

§ 87.  Collotype  Colour  Process . 161 

§ 88.  Sanger-Shepherd’s  Imbibition  Process  . . . .163 

| 89.  Pinatype  Process 166 

§ 90.  Colour  Carbon  and  other  Processes 170 

§ 90a.  Raydex  Colour  Process 171 


CHAPTER  XI 

COLOUR  PRINTING  FROM  SINGLE-PLATE  TRANSPARENCIES 


§ 91.  Uto-color  Printing 179 

§ 92.  Theory  of  Bleach-out  Process 180 

§ 93.  Permanent  and  Fugitive  Colours 182 

§ 94.  Details  of  Bleach-out  Process 186 

§ 95.  Nature  of  Dyes 189 

§ 96.  Uto-color  Paper 192 

§ 97.  Practical  Details  of  Printing  on  Rapid  Uto  Paper  . . 195 

§ 98.  Methods  of  Improving  the  Print 198 

| 99.  Fixing  the  Uto-color  Prints 199 

§ 100.  Uto-color  Stripping  Paper 200 

§ 101.  Uto  Lahtern  Slides  . 202 


CHAPTER  XII 

KINEMATOGRAPHY  BY  MEANS  OF  COLOURED  LIGHTS 


§ 102.  Projection  of  Kiuematograph  Pictures  in  Colour  . . 203 

§103.  The  Urban-Smith  “ Kinemacolor  ” Process  . . . 203 

§ 104.  The  Kinemacolor  Camera  208 

§ 105.  The  Kinemacolor  Projector  209 

§ 105a.  Gaumont’s  Method  of  Colour  Cinematography  . . 212 


CONTENTS 


Xlll 


CHAPTER  XIII 

COLOUR  PHOTOMICROGRAPHY 

PAGE 

§ 106.  Discussion  as  to  the  Advantages  of  Different  Methods  . 214 

§ 107.  Low  Power  Photomicrography 215 

§ 108.  Illumination 218 

§ 109.  Methods  for  Finding  the  Position  of  Object  and  Image — 

Magnification 218 

§ 110.  Exposure 221 

§ 111.  Factors  which  Influence  Exposure  .....  222 

§ 112.  High-power  Photomicrography 225 

CHAPTER  XIV 

ART  IN  COLOUR  PHOTOGRAPHY 

§ 113.  What  constitutes  Art  ? 241 

!114.  Primary,  Secondary,  and  Tertiary  Colours  . . .241 

115.  Hue,  Tint,  and  Shade 244 

116.  How  to  produce  Shadows  in  Colours 245 

117.  General  Hints  as  to  Colour 247 

118.  Shadows 250 

119.  Choice  of  Subjects 251 

§ 120.  Portrait  Photography 254 

§ 121.  Backgrounds 256 

APPENDIX 

Theories  of  Colour  Perception 259 

1.  Table  of  Exposures  for  Separate  and  Combined  Colour  Plates  263 

2.  „ ,,  Sunset  Exposures 265 

3.  ,,  ,,  Additive  Colour  Effects  (Colour  Synthesis)  . . 266 

4.  ,,  ,,  Relative  Brightness  of  Spectrum  ....  266 

5.  ,,  ,,  Slowest  Exposures  for  Sharpness  ....  267 

6.  ,,  ,,  Factor  Numbers 267 

7.  Instructions  for  developing  Autochrome  Plates  . . . 270 

8.  Lumibre’s  Improved  Formula,  1908  271 

9.  ,,  Graduated  Developer,  1910 272 

9a.  Professor  Namias’s  Method  of  Developing  Autochromes  . 274 

10.  Other  Developers 275 

11.  Intensification  Formulae 276 

12.  Reduction  Formulae  ........  277 

13.  Instructions  for  Developing  Omnicolore  Plates  . . . 278 

14.  „ ,,  ,,  Dufay  Plates  ....  280 

15.  ,,  ,,  ,,  Paget  Plates  ....  282 

16.  Elimination  of  Green  Spots 283 

17.  Devices  for  protecting  Slide  from  Heat 284 


XIV 


CONTENTS 


PAGE 

18.  Sensitizing  Colour  Plates 284 

19.  Colour  Screen  Filters  for  Monochromatic  Light  . . . 285 

20.  A.  B.  Hitchin’s  Developer 286 

21.  Metric  Equivalent  Tables 287 

22.  English  and  Foreign  Sizes  of  Plates 290 

23.  Comparative  Plate  Speeds 290 

24.  Wave  Lengths  of  Visible  Spectrum 291 

25.  Thermometric  Scales 292 

26.  List  of  Names  mentioned  in  Text 293 

INDEX 295 


LIST  OF  PLATES 


PLATE 

I.  July  at  Hampton  Court,  reproduced  from  a 

Raydex  Photograph  ....  Frontispiece 

FACING  PAGE 

II.  Normal  Fundus  (Background  of  the  Eye)  of  a 

Man  forty  years  old 43 

III.  Coloured  Oil  Globules  in  the  Retina  of  a 
Tortoise,  Domestic  Fowl,  and  Pigeon,  compared 
with  Appearance  of  the  Starch  Grain  Layer 

in  an  Autochrome  Plate 45 

IV.  Spectrum  of  Total  Colour-Blindness  ...  51 

Y.  Section  of  a Lippmann  Photographic  Film  . . 65 

VI.  Charts  of  Additive  Lights  and  Subtractive 

Colours 68 

VII.  “Thames”  and  “Omnicolore”  Screens  X 100  . 76 

VIII.  “ Dioptichrome  ” and  “ Krayn  ” Screens  X 100  . 78 

IX.  Starch  Grains  of  Autochrome  magnified  . . 84 

X.  “Autochrome”  and  “Warner  Powrie  ” Screens 

X 100 86 

XI.  Kinemacolor  Negative  and  Positive  Films  . . 208 

XII.  The  Kinemacolor  Projector,  showing  Colour 

Filter  in  Position 210 

XIII.  Apparatus  arranged  for  Kinemacolor  Projection  212 

XIV.  Another  View  of  the  Central  Part  of  the 

Apparatus  for  Photomicrography  . . . 228 


PHOTOGRAPHY  IN  COLOURS 


CHAPTER  I 

TH£  natuee  of  light  and  coloue 

§ 1.  Sources  of  Light. — When  a body  is  raised  to  a 
high  temperature  it  becomes  incandescent,  and  sets  up 
vibrations  or  waves  in  all  directions  in  the  ether,  which 
waves,  impinging  on  the  back  of  our  eyes,  give  rise  to 
the  sensation  of  light.  With  the  exception  of  Fluore- 
scence, Phosphorescence,  and  a few  other  sources  of 
energy,  which  need  not  concern  us  here,  all  light  has 
its  source  in  bodies  which  are  in  a condition  of  white 
heat  or  incandescence.  Thus  we  see  either  by  reason  of 
the  white  hot  matter  of  the  sun  and  stars,  or  by  means 
of  artificial  sources,  such  as  the  incandescent  particles 
of  carbon  in  a candle  or  gas- flame,  or  the  white-hot  fila- 
ments in  the  electric  bulb.  It  is  by  no  means  necessary 
that  the  source  of  light  should  be  seen.  Thus  most  of 
the  objects  around  us,  such  as  the  moon,  trees,  houses, 
etc.,  are  rendered  visible  by  the  light  reflected  from 
them,  which  light  can  invariably  be  traced  to  one  of 
the  sources  just  named. 

§ 2.  Nature  of  Light. — The  way  in  which  objects 
are  seen,  and  what  constitutes  light,  are  problems  that 
have  occupied  the  mind  of  man  ever  since  he  became 

B 


2 


PHOTOGRAPHY  IN  COLOURS 


a thinking  animal.  The  primeval  savage  must  have 
observed  that  the  gilding  of  the  sky,  and  the  creeping 
darkness,  accompanied  the  setting  sun.  'He  must  have 
wondered  why  his  spear  appeared  bent  when  he  thrust 
it  into  the  pool,  and  why  his  shadow  became  shorter 
as  the  sun  rose  in  the  heavens. 

The  Greeks,  who  wasted  far  too  much  of  their  time 
in  fruitless  speculation  without  testing  their  theories 
by  experiment,  imagined  that  light  was  something 
that  passed  from  the  eye  to  the  object  seen,  in  the 
form  of  some  invisible  tentacle  or  emanation.  This 
theory  was  abandoned  for  the  one  that  sight  was  due 
to  the  impact  of  an  infinite  number  of  minute  luminous 
corpuscles  which  stream  out  from  every  visible  object 
at  an  immense  velocity,  entering  the  eye  and  giving 
rise  to  vision  by  their  impact  against  the  retina,  much 
in  the  same  way  that  the  particles  or  corpuscles  are 
seen  to  dart  out  of  a speck  of  Radium  when  ob- 
served through  a Spinthariscope.  The  fact  that  light 
could  pass  through  glass  and  other  transparent  bodies, 
was  accounted  for  by  the  supposition  that  these  cor- 
puscles were  so  minute  that  they  could  pass  between 
the  molecules  of  any  transparent  substance.  This 
view,  which  is  known  as  the  Corpuscular  Theory,  was 
held  by  Newton ; but  Huyghens,  his  contemporary, 
was  the  first  to  propound  the  wave  theory  of  light. 
Unfortunately  the  wave  theory  was  overshadowed  by 
the  great  name  of  Newton,  and  it  was  not  generally 
accepted  until  the  experiments  of  Thomas  Young  and 
Fresnel  established  it  on  a solid  basis  at  the  beginning 
of  the  nineteenth  century. 

In  order  to  understand  the  wave  theory,  we  are 


THE  NATURE  OF  LIGHT  AND  COLOUR  3 


obliged  to  assume  the  existence  of  an  omnipresent 
medium  which  is  called  the  Ether. 

§ 2a.  Ether. — This  is  supposed  to  be  a perfectly 
elastic,  frictionless,  and  imponderable  medium  which 
occupies  the  entire  space  throughout  the  visible  universe, 
surrounding  and  penetrating  the  molecules  and  atoms 
of  which  all  matter  is  composed.  It  is  further  supposed 
to  be  absolutely  motionless,  and  also  to  possess  enor- 
mous rigidity,  so  that  no  amount  of  force  is  able  to 
rend  or  displace  it.  Its  chief  property  is  that  of  pro- 
pagating vibrations  or  waves  which  travel  transversely 
to  the  line  of  force.  These  waves  radiate  in  every 
direction  like  the  circumference  of  an  ever-expanding 
globe  when  considered  as  a whole,  although  any  single 
wave  propagated  from  the  source  of  origin  will  continue 
to  generate  waves  in  a straight  line  to  an  infinite  dis- 
tance without  any  appreciable  loss  of  energy.  It  is 
further  supposed  that  gravity,  or  the  action  of  one  body 
on  another  at  a distance,  together  with  the  various 
forms  of  energy,  such  as  light,  radiant  heat,  electricity, 
magnetism,  Hertzian  waves,  etc.,  are  all  manifestations 
of  the  same  wave  motion,  and  that  these  waves  are 
due  to  stresses  in  the  ether,  which  are  propagated 
through  that  medium  with  immense  velocity.  These 
light  waves  are  supposed  to  cause  movements  in  the 
ether,  which  are  so  small  and  so  rapid  that  the  latter 
behaves  like  an  elastic  solid.  The  familiar  illustration 
of  waves  formed  in  a still  pond  may  help  the  reader  to 
grasp  the  idea.  If  the  surface  of  a smooth  pond  be 
strewn  with  corks,  and  a stone  be  thrown  into  it,  a 
series  of  concentric  circular  waves  will  be  generated 
which  will  spread  further  and  further  to  the  shore,  but 


4 


PHOTOGRAPHY  IN  COLOURS 


the  water  itself  will  not  travel,  as  will  be  shown  by  the 
corks,  which,  although  they  bob  up  and  down,  never- 
theless remain  where  they  were  originally  placed.  In 
like  manner  the  ether  remains  unmoved,  save  that 
minute  stresses  are  caused  which  propagate  impulses 
with  the  velocity  of  light  in  all  directions,  and  which 
follow  one  another  with  inconceivable  rapidity,  the 
frequency  amounting  to  many  billions  of  waves  per 
second. 

§ 3.  Nature  of  Ether  Waves. — The  energy  engen- 
dered by  the  source  of  light  sets  up  vibrations  transversely 
to  the  direction  of  motion.  These  waves  vary  enor- 
mously in  length,  i.e.  in  the  distance  between  the  crests 
of  successive  waves.  Thus  many  kinds  of  electrical 
vibrations,  especially  Hertzian  waves,  vary  from  two 
or  three  mm.  to  several  miles  in  length,  whereas  the 
waves  which  we  perceive  with  our  eyes  vary  between 
768  /z/z  (millionths  of  a mm.)  for  the  dark  red  rays,  to 
375  /z/z  (or  slightly  less  than  half  the  former  length) 
for  the  violet  rays.  By  passing  the  light  through  a 
fluor  spar  prism,  waves  of  only  200  /z/z  can  be  rendered 
visible,  and  by  the  additional  help  of  photography  and 
the  use  of  a vacuum  camera,  waves  of  a still  shorter 
length,  viz.  100  /z/z,  can  be  registered  on  the  photo- 
graphic plate.  Waves  which  exceed  768  /z/z  in  length 
are  invisible  to  the  unaided  eye,  and  are  called  the 
infra-red  or  heat-rays  of  the  spectum.  These,  although 
they  cannot  be  seen  by  the  eye,  may  be  regarded  by 
means  of  an  instrument  called  a Bolometer.  Those 
rays  which  are  shorter  than  375  /z/z  belong  to  the 
ultra-violet  end  of  the  spectrum,  and  are  likewise 
invisible. 


THE  NATURE  OF  LIGHT  AND  COLOUR  5 


§ 4.  Brief  Outline  of  the  Wave  Theory. — Every 
part  of  a source  of  light  generates  a wave,  which  travels 
in  every  direction.  Let  one  of  these  parts  L be  an  in- 
candescent point  of  light.  This  will  form  the  centre  of 
a minute  sphere  whose  diameter  is  equal  to  a wave 
length  (A).  Every  point  in  the  circumference  of  this 
sphere  will  at  once  form  a fresh  centre  of  disturbance 
and  will  generate  a new  sphere,  and  this  again  another, 
and  so  on.  Now  every  one  of  these  tiny  spheres  may 
be  supposed  to  lie  side  by  side,  each  overlapping  its 
fellow,  forming  tangents  to  points  on  their  combined 
circumference,  which  points  are  situated  on  imaginary 
radii  from  the  original  point  of  light. 

Since  each  fresh  sphere  generated  from  the  smaller 
sphere  behind  it,  has  for  its  centre  a point  on  the  same 
radius,  tangents  to  points  on  their 
combined  circumference  will,  if 
taken  collectively,  form  a wave 
front  ( a , l,  c,  d , e).  Now,  as  the 
centre  of  each  sphere  lies  on  the 
wave  front  of  the  sphere  behind 
it,  the  diameter  of  each  sphere  is 
clearly  equal  to  a wave-length. 

Each  successive  wave  front  (Alf  A2) 

(Fig.  1)  may  thus  be  considered 
as  the  crest  of  a wave,  and  the  space  between  it  and 
the  next  wave  front  (A^,  A^)  as  the  trough  of  a wave. 
Thus  light  waves  advance  to  form  an  ever  enlarging 
sphere,  and  the  tangent  to  this  sphere  represents  the 
wave  front.  Since  the  centre  of  each  tiny  sphere  lies 
on  a common  radius,  light  waves  may  be  said  to 
advance  in  straight  lines  from  the  source,  just  as  waves 


6 


PHOTOGRAPHY  IN  COLOURS 


may  be  said  to  travel  along  a stretched  rope  when  it  is 
shaken.  Such  a straight  line  of  force  is  termed  a ray. 
A small  collection  of  such  rays  is  termed  a pencil,  and 
a larger  one  a bundle , or  if  divergent  from  a point  of 
light  a cone  of  rays.  For  some  purposes  it  will  be 
found  more  convenient  to  refer  to  light  in  terms  of  its 
wave  front,  but  for  most  purposes,  and  more  especially 
in  geometrical  optics,  the  direction,  and  not  the  wave 
front,  has  to  be  considered,  and  may  conveniently  be 
represented  by  a straight  line. 

We  must  also  distinguish  between  a train  of  waves 
and  a single  wave.  If  we  take  the  familiar  example  of 
the  stone  dropped  into  the  pond,  we  notice  that  the 
front  wave  gets  feebler  and  feebler  as  it  spreads  out 
towards  the  shore,  while  the  main  body  or  train  of  the 
waves  moves  on  undiminished.  In  free  ether,  as  there 
is  no  resistance,  both  the  train  and  the  single  waves 
travel  unimpaired  to  infinity,  but  in  a refracting 
medium  such  as  glass  or  water,  the  single  waves 
become  rapidly  exhausted.  Thus  a single  wave  cannot 
pass  through  a thin  microscope  cover-slip,  much  less 
through  a plate  of  glass.  It  has  been  found  that  in 
Bisulphide  of  Carbon  a wave  loses  N 0f  its  amplitude 
for  each  wave-length,  and  that  after  14  or  15  wave- 
lengths the  wave  has  quite  died  out.  The  train  also 
suffers  in  a dense  medium,  not  only  by  becoming 
slower  in  direct  proportion  to  its  refractive  index,  but 
by  becoming  exhausted  as  well.  The  train  of  waves 
lose  about  ~ of  their  amplitude  for  each  fathom  of 
water  penetrated,  so  that  light  cannot  pass  through  a 
thickness  of  50  or  60  fathoms.  Objects  below  that 
depth  being  in  absolute  darkness.  Of  course  a much 


THE  NATURE  OF  LIGHT  AND  COLOUR  7 


less  thickness  of  glass  would  suffice  to  obstruct  all  light. 
The  difference  between  trains  of  waves  and  single 
waves  may  be  illustrated  by  a regiment  of  soldiers  who 
are  taking  a fortress  by  assault.  The  front  men  drop 
down  one  by  one,  but  the  main  body  of  men,  which 
represents  the  train  of  waves,  rushes  on  unhampered  to 
the  goal. 

§ 5.  Electro  - magnetic  Theory  of  Light. — Al- 
though it  is  generally  accepted  that  light  is  due  to 
transverse  vibrations  set  up  in  the  ether,  it  is  extremely 
doubtful  if  these  vibrations  are  propagated  by  succes- 
sive portions  of  the  ether  which  set  each  other  in  motion, 
since  this  force  can  be  resolved  into  two  components, 
one  of  which  will  be  found  to  be  in  the  direction  of  the 
wave-front,  and  will  consequently  set  up  longitudinal 
vibrations.  But  there  is  no  evidence  whatever  to 
prove  that  longitudinal  waves  can  be  transmitted  in 
ether  at  all. 

Clerk  Maxwell,  in  his  electro-magnetic  theory  of 
light,  got.  over  this  theory  by  supposing  that  the 
waves  do  not  necessitate  any  change  in  the  position  of 
the  ether  particles,  but  that  they  cause  periodic  and 
almost  instantaneous  changes  in  their  electro-magnetic 
condition.  Faraday  showed  that  the  ether  was  a 
perfect  dielectric  (non-conductor)  which,  like  all 
dielectrics,  is  capable  of  polarisation.  He  traced  all 
electric,  magnetic,  and  optical  phenomena  to  stresses 
in  the  ether.  In  fact  we  can  classify  these  ether 
stresses  according  to  the  size  of  the  waves  and  their 
method  of  formation,  so  that  we  are  able  to  group  them 
into  light  waves,  heat  waves,  and  electric  waves.  By 
means  of  the  electro-magnetic  theory  Clerk  Maxwell 


8 


PHOTOGRAPHY  IN  COLOURS 


established  the  close  relationship  between  light  and 
electricity  on  a mathematical  basis,  and  it  remained 
for  Hertz,  the  brilliant  pupil  of  Helmholtz,  to  prove 
experimentally  that  electric  waves  were  not  only  propa- 
gated in  straight  lines,  but  were  capable  of  reflection, 
refraction,  and  polarisation.  Further,  the  velocity  of 
electric  waves  was  measured  and  found  to  be  identical 
with  that  of  light.  In  a word  Hertz  demonstrated 
by  experiment  the  truth  of  Clerk  Maxwell’s  assumption, 
that  light  was  nothing  more  nor  less  than  an  electro- 
magnetic effect  in  the  dielectric  and  polarisable  ether. 
Whenever  electric  waves  pass  through  a dielectric  we 
get  a displacement  at  right  angles  to  the  direction  of 
the  wave,  exactly  as  we  find  to  be  the  case  with  light- 
waves in  the  ether. 

The  phenomena  of  electrolysis,  kathode  rays, 
Becquerel  rays,  and  especially  the  Zeeman  and  the 
Kerr  phenomena,  all  go  to  prove  that  electricity  is 
merely  another  condition  of  light ; and  physicists  seem 
to  be  more  or  less  agreed  that  the  electrical  phenomena 
are  due  to  the  propulsion  of  electrons,  which  are  the 
smallest  particle  of  any  body  we  know  of,  and  which 
appear  to  be  a kind  of  transition  between  matter  and 
ether.  Whenever  an  electron  is  set  free,  it  is  ac- 
companied by  an  electric  current  which  sets  up  a 
stress  in  the  ether,  and  generates  a series  of  intensely 
rapid  alternating  positive  and  negative  electric  polari- 
sations or  displacements  whereby  the  electro-magnetic 
waves  which  exhibit  the  phenomena  of  light  are 
produced. 

The  reader  must  dismiss  the  popular  idea  that 
electricity  is  a fluid  which  passes  through,  or  is  dis- 


THE  NATURE  OF  LIGHT  AND  COLOUR  9 


tributed  around,  a conductor  or  wire.  Electricity  (or 
light)  charges  the  surrounding  ether  with  energy 
which  manifests  itself  in  the  form  of  waves.  When- 
ever an  electric  conductor  is  said  to  be  charged,  it  is  not 
the  conductor  or  wire  which  is  charged , hut  the  ether  around 
it , and  a flow  of  electricity  merely  implies  a flow  or 
distribution  of  energy  through  the  electric  or  electro- 
magnetic field. 

§ 6.  The  Nature  of  White  Light. — The  classical 
experiments  of  Newton  apparently  showed  that  white 
light  consisted  of  a mixture  of  all  the  colours,  and  that 
a prism  merely  separates  them  out  into  the  seven 
primaries.  We  find  this  stated  as  a fact  in  the  majority 
of  text-books  even  to-day.  If,  however,  we  examine 
the  question  a little  more  closely,  we  find  the  solution 
is  by  no  means  so  simple.  It  is  evident  that  white 
light  consists  of  an  infinite  number  of  trains  of  waves 
each  having  a different  wave-length.  Hence  the 
stresses  set  up  in  the  ether  must  be  the  resultant  of  all 
these  numberless  trains,  and  consequently  they  can 
have  no  regularity  of  sequence.  On  the  other  hand, 
we  know  that  a prism  or  grating  will  separate  white 
light  into  a number  of  distinct  colours,  and  a pure 
colour  can  only  be  produced  by  a regularity  in  the 
sequence  of  the  train  waves.  Hence  arises  the  question, 
does  the  prism  or  grating  separate  out  the  waves  into 
orderly  trains  of  sequence,  or  were  they  originally 
present  in  the  white  light  ? As  Professor  Wood  rightly 
puts  it,  if  the  regular  wave  trains  were  manufactured 
by  the  prism,  was  Newton’s  discovery  really  a discovery 
after  all  ? 

The  answer  to  this  question  is  an  extremely 


IO 


PHOTOGRAPHY  IN  COLOURS 


complicated  one,  and  it  is  very  difficult  in  an  elementary 
text-book  to  make  the  reasoning  intelligible  to  the  ordi- 
nary non -mathematical  reader.  Many  Physicists  insist 
that  white  light  from  a source  consists  of  regular  trains 
of  waves,  because  interference  fringes  can  be  obtained 
from  it,  either  by  employing  two  slits,  or  by  means  of 
Fresnel’s  mirrors  ( i.e . by  reflection  from  two  mirrors 
placed  at  an  extremely  wide  angle  to  one  another). 
But  Schuster  has  shown  that  the  interference  is  due  to 
a physiological  peculiarity  of  the  eye.  We  may  consider 
that  all  the  retinal  rods  and  cones  are  toned  to  the 
three  primary  colours  in  the  same  way  that  musical 
instruments  or  gas  flames  enclosed  in  glass  tubes 
respond  to  certain  vibrations  or  multiples  of  them. 
This  conception  is  quite  a different  thing  from  Helm- 
holtz’s idea  that  the  cones  were  divided  into  three 
groups,  each  one  responding  to  one  of  the  three  primary 
colours,  and  not  to  the  other  two.  According  to 
Schuster’s  theory,  the  retinal  elements  have  a period 
of  their  own,  so  that  they  respond  to  certain  wave- 
lengths and  not  to  others.  Now  suppose  the  white 
light  to  consist  of  a series  of  pulses  which  result  from 
an  immense  number  of  simple  harmonic  waves  differing 
from  one  another  by  insensible  gradations.  When 
two  pulses  strike  a cone  one  after  the  other  the  effect 
will  depend  on  the  interval  between  the  two  impacts. 
If  the  first  impact  sets  the  cone  vibrating  and  the 
second  shock  arrives  before  the  first  has  died  out,  in- 
terference will  obviously  take  place,  the  second  impact 
either  intensifying  or  annulling  the  first  one,  according 
to  whether  the  waves  are  in  the  same  phase  or  a 
different  one.  In  this  way  interference  fringes  can 


THE  NATURE  OF  LIGHT  AND  COLOUR  II 


readily  be  seen.  The  molecules  in  minute  particles  of 
silver  compounds  in  the  film  likewise  have  their  periods 
of  resonance  and  can  be  set  vibrating,  and  Schuster 
has  even  succeeded  in  photographing  the  fringes 
formed. 

We  may  perhaps  explain  the  whole  matter  in  a very 
simple  way : — Imagine  a post  bag  filled  with  letters 
from  London  and  forwarded  to  Liverpool.  The  letters 
are  all  mixed  up  anyhow.  This  represents  our  waves 
of  white  light.  On  arriving  at  the  G.P.O.  in  Liverpool 
some  are  sorted  for  the  United  States,  some  for  local 
distribution,  some  for  the  Docks,  and  so  on.  This  sort- 
ing corresponds  to  the  effect  of  a prism  or  grating 
which  manufactures  the  colours  by  sorting  out  the 
wave-lengths  into  regular  periods  corresponding  to  their 
respective  wave-lengths.  But  suppose  that  the  United 
States  letters  were  held  back  at  the  post-office,  and 
then  added  to  others  going  in  the  same  direction  so 
that  they  could  all  arrive  at  New  York  in  a number  of 
big  sacks.  This  would  represent  the  way  in  which  the 
pulses  follow  on  in  the  eye  by  the  fusion  of  the  periods 
of  resonance  due  to  the  first  pulse  with  that  of  the 
next  arrival. 

§ 7.  The  Cause  of  the  Sensation  of  Colour. — The 

proof  that  colour  is  due  to  the  frequency  of  the  vibrations, 
and  not  to  their  wave-lengths,  can  readily  be  shown  by 
their  analogy  to  sound  waves.  Our  sensations  of  light 
and  colour  are  due  to  an  enormously  rapid  succession 
of  waves,  or  taps,  on  the  ends  of  the  rods  and  cones  of 
the  retina,  which  we  can  perceive  provided  their 
frequencies  lie  between  certain  limits.  Exactly  the 
same  thing  happens  to  the  ear  in  the  case  of  sound 


12 


PHOTOGRAPHY  IN  COLOURS 


waves.  These  latter  vibrations  are  termed  the  pitch  of 
the  note,  and  we  can  hear  them,  provided  they  amount 
to  not  less  than  39  or  more  than  36,000  vibrations  a 
second,  when  they  tap  the  hair  cells  of  our  auditory 
nerve,  which  is  situated  inside  the  inner  ear.  Now, 
owing  to  the  slow  rate  of  our  auditory  vibrations,  it  is 
quite  easy  to  show  that  the  pitch  or  musical  colour  is 
determined  by  the  frequency  of  its  vibrations,  and  not 
by  the  note,  or  musical  wave-lengths.  From  this  we 
may  infer  that  colour  is  due  to  the  frequency  of  the 
waves  and  not  to  their  wave-length. 

Again,  we  know  that  the  velocity  of  light  is  retarded 
when  passing  from  air  through  a denser  medium  such 
as  water  or  glass.  If,  therefore,  we  allow  a colour  of 
known  wave-length,  such  as  sodium  light,  to  pass 
through  the  slit  of  the  spectroscope,  and  place  in  its 
path  a plate  of  glass,  it  should  cause  an  alteration  in 
the  colour  towards  the  blue  end  of  the  spectrum,  owing 
to  the  reduction  of  the  speed.  But  no  such  change  can 
be  observed.  Hence,  in  the  equation  Y = X/,  where 
Y = velocity  of  light,  X = wave-length,  and  / = fre- 
quency, if  we  reduce  Y in  causing  the  light  to  pass 
through  the  glass  plate,  we  must  either  reduce  X or  / to 
satisfy  the  equation.  But,  as  we  have  just  seen,  colour 
does  not  change  when  light  passes  through  a denser 
medium,  therefore  colour  must  be  due  to  frequency  and 
not  to  wave-length. 

§ 8.  How  Colour  is  Produced. — The  chief  source 
of  colour  in  objects  is  selective  absorption  of  portions 
of  the  constituents  of  white  light  reaching  it,  and  the 
scattering  of  the  residue,  which  latter  causes  the  colour 
of  the  objects  which  we  perceive. 


THE  NAT  ERE  OF  LIGHT  AND  COLOUR  1 3 


In  the  same  way,  when  white  light  passes  through  any 
substance  some  of  its  components  are  absorbed,  while 
the  rest  of  the  light  which  emerges  is  coloured  in 
consequence.  An  object  is  only  colourless  when  white 
light  passes  through  it  unchanged,  the  portion  ab- 
sorbed consisting  of  trains  of  waves  of  every  refrangi- 
bility. 

When  light  reaches  the  surface  of  any  non-trans- 
parent object,  it  either  penetrates  a greater  or  less 
distance  beneath  the  surface,  where  it  is  reflected  and 
scattered  by  the  layers  and  the  surface,  and  thus 
reaches  the  eye;  or  it  rebounds  directly  from  the 
surface  from  whence  the  light  is  scattered  by  its  in- 
equalities. In  the  latter  case  the  light  remains 
unchanged,  but  in  the  former  case  some  of  the  con- 
stituents of  the  light  are  absorbed,  the  remainder  being 
coloured  light  which  gives  the  characteristic  hue  or 
colour  to  the  substance.  For  example,  a rose  appears 
red  because  the  white  light  falling  on  it  penetrates  the 
cells  of  the  petals  which  absorb  the  green  and  some  of 
the  blue  constituents  of  the  light,  while  all  the  red, 
some  of  the  blue,  and  some  of  the  unchanged  light 
is  reflected  back  and  scattered  at,  or  beneath,  the 
surface,  giving  the  impression  to  the  eye  of  a pink 
or  red  flower.  It  is  because  substances  with  few 
exceptions  are  not  homogeneous  (that  is  of  uniform 
density  throughout)  that  internal  reflections  and  re- 
fractions occur  which  project  the  unabsorbed  light  into 
space,  and  thus  give  rise  to  colours.  To  this  action, 
according  to  T.  E.  Goodall,  the  luminous  shadows 
which  appear  on  the  human  face  are  due.  When  a 
faint  shadow  is  thrown  upon  the  face,  a portion  of  the 


14 


PHOTOGRAPHY  IN  COLOURS 


light  penetrates  the  skin  and  is  reflected  back  by  the 
reddish  tinge  of  the  layers  beneath.  It  is  to  this  that 
the  warmth  of  tone  and  transparency  of  the  shadows 
are  due.  In  the  portraits  by  the  old  masters  the  painter 
seized  upon  this  effect  which  gave  such  life  to  the  por- 
traits. To  those  who  use  face  powders  this  reflective 
action  of  the  skin  is  prevented,  hence  the  corpse-like 
shadows  which  are  seen  instead  of  the  beautiful  pearly 
luminosity  of  a fresh  untouched  face. 

To  say,  as  some  text-books  do,  that  bodies  and  pig- 
ment particles  reflect  certain  colours  more  strongly 
than  others  is  incorrect.  Light  reflected  from  a homo- 
geneous medium  is  always  white. 

The  difference  between  a liquid  and  its  froth  is  a 
striking  example  of  internal  reflections  and  scattering 
of  the  light.  Thus,  a block  of  ice  has  a pale  cobalt- 
blue  colour,  but  when  powdered  it  is  quite  white.  In 
the  same  way  the  crest  of  a wave  blown  about  by  the 
wind,  or  the  froth  of  beer  is  white.  Almost  any  crys- 
talline substance,  whatever  its  colour,  will  become 
white,  or  nearly  white,  if  sufficiently  finely  powdered, 
or  condensed  in  the  form  of  snow.  This  can  be  readily 
shown  in  the  case  of  sulphate  of  copper,  or  bichromate 
of  potassium  crystals.  In  all  the  above  cases  the  light 
which  falls  on  the  liquid  or  powders  suffers  reflection 
or  refraction  at  the  surface  of  each  particle  or  bubble 
on  which  it  falls.  These  rays  undergo  innumerable 
reflections,  and  the  light  is  scattered  in  every  direction. 
As  very  little  light  is  absorbed  all  the  colours  are 
equally  reflected,  and  at  the  same  time  the  intensity  of 
the  intromitted  rays  is  almost  unimpaired ; the  result 
being  that  the  surface  of  the  body  appears  dazzling 


THE  NATURE  OF  LIGHT  AND  COLOUR  1 5 


white.  Thus  a cloud  when  at  a great  elevation  consists 
of  minute  crystalline  ice-particles  (cirrhus  clouds)  which 
particles  are  so  small  in  comparison  with  their  areas, 
that  gravity  has  little  or  no  effect  on  them,  and  they 
form  brilliant  white  clouds.  If  the  pressure  of  the 
atmosphere  diminishes  the  clouds  sink,  and  the  par- 
ticles melt  and  run  together.  This  causes  the  light  to 
become  more  absorbed,  and  they  form  cumuli.  But 
since  the  light  is  not  selectively  absorbed  they  appear 
grey.  If  the  clouds  condense  still  more,  the  light 
is  largely  absorbed,  and  they  form  dark  nimbi  or  rain 
clouds. 

If  in  the  case  of  bodies  or  liquids  the  substance  has 
a selective  power  of  absorption,  the  emittent  rays  are 
deprived  of  some  of  their  constituents,  and  the  body 
appears  coloured.  In  some  cases — as,  for  instance, 
the  powders  above  mentioned — the  light  is  reflected 
very  close  to  the  surface,  and  does  not  pass  through 
enough  of  the  substance  to  allow  of  any  selective 
absorption. 

§ 9.  Coefficient  of  Absorption  and  Transmission 
of  Light. 

Lambert’s  Law. — Lambert  found  that  the  amount 
of  light  which  passed  through  a coloured  filter  or  glass 
plate  diminished  in  geometrical  progression  as  the 
thickness  increased  in  arithmetical  progression. 

Thus,  if  we  superpose  a number  of  layers  of,  say, 
one  millimetre  thick,  as  the  thickness  of  the  layers 
increases  in  the  ratio  of  1,  2,  3,  4,  5,  etc.,  the  intensity 
of  the  emergent  light  will  diminish  in  the  ratio  of  1, 
b e^c*>  80  that  if  we  place  a series  of  identical 

coloured  glasses  of  the  same  thickness  one  after  another 


PHOTOGRAPHY  IN  COLOURS 


l6 

in  front  of  the  lens  in  a camera,  the  exposure  would  be 
in  the  ratio  of  1,  2,  4,  8,  16  times,  as  the  thickness 
traversed  by  the  light  was  increased  from  1 mm.  to 
2,  3,  4,  and  5 mm. ; or,  to  put  it  in  another  way,  the 
exposure  varies  as  the  logarithm  of  the  thickness.  Thus, 
let  I = intensity  of  the  incident  light,  and  la  = intensity 
after  transmission,  through  a unit  of  thickness,  ( t ) being 
the  coefficient  of  transmission.  Then  for  one  unit  of 
thickness  (or  t ) we  have  an  intensity  = la,  for  2 1 an 
intensity  I a.  a = la2,  for  3tla  . a . a =Ia3  . . . and  for 
xt  an  intensity  of  I ax. 

As  a rule,  the  colour  merely  increases  in  depth  with 
the  thickness  of  the  material,  but  there  are  exceptions 
to  this.  For  example,  the  well-known  liqueur  Creme 
de  Menthe,  which  is  sold  in  Florence  flask-shaped 
bottles,  when  held  up  in  front  of  a small  source  of 
light,  such  as  an  incandescent  globe,  appears  green  in 
the  neck,  but  red  in  the  spherical  part,  and  a yellow 
transition  at  one  spot  between  the  two.  The  explana- 
tion of  this  is  as  follows : — Let  lg}  Ir  be  the  initial  in- 
tensities for  the  green  and  red  colours,  and  ag , ar  their 
coefficients  of  transmission.  Then,  from  what  we  have 
just  stated,  the  intensities  of  the  colours  after  trans- 
mission will  be  I ga  * and  I rarx.  Now,  if  the  thickness 
of  the  plate  is  small,  there  will  not  be  much  difference 
between  ag  and  ar,  and  if  1^  is  much  greater  than  lr, 
and  ar  only  slightly  greater  than  ag , then  lg  (that  is,  the 
green  rays)  will  greatly  predominate,  and  the  liquid 
will  appear  green.  If  we  increase  the  thickness  of 
the  layer  it  is  obvious  that  ar  will  greatly  exceed  ag 
until  I gag  = I raf,  when  the  two  intensities  coincide, 
and  the  result  will  be  yellow.  Thus,  suppose  lg  = 100, 


THE  NATURE  OF  LIGHT  AND  COLOUR  1 J 

Ir  = 50,  ag  = 5 and  ar  = 8,  then,  since  I/t*  = I rarx,  by 
changing  it  into  logarithms  we  may  write 

g _ !og  l,  ~ log  I. 

log  ar  — log  ag 

log  100  - log  50  _ 2 — 1,7  _ 0,3 
0r  log  8 - log  5 “ 0,9  - 0,7  - 0,2 

Therefore  x = 1,  thus  determining  the  thickness  at 
which  the  two  intensities  are  equal  and  the  liquid 
appears  yellow.  If  the  thickness  be  still  further  in- 
creased, I rarx  will  be  greater  than  I gag,  and  the  liquid 
will  appear  red  when  held  up  to  the  light. 

A still  better  example  of  di-chromatism  can  be  made 
by  dissolving  “ Brilliant  Green  ” and  Naphthaline  Yel- 
low in  hot  Canada  Balsam,  and,  when  cool,  squeezing 
the  mixture  between  two  glass  plates  ground  in  the 
form  of  a thin  prism.  The  thin  end  of  the  wedge  will 
then  appear  green,  and  the  thick  edge  red,  and  some- 
where between  the  two  ends  where  the  transmission 
is  the  same  for  both,  yellow. 

When  a white  flower  is  placed  in  coloured  light,  it 
assumes  the  colour  of  the  light  thrown  on  it,  since 
it  does  not  absorb  any  special  colour,  but  reflects  all 
light  equally.  In  the  same  way  most  coloured  flowers 
will  assume  the  colour  of  the  light  in  which  they  are 
placed,  but  some  flowers  absorb  so  much  of  the  incident 
light  that  they  appear  nearly  or  quite  black.  Thus  a 
red  poppy  or  a red  tulip  will  appear  a brilliant  red  in 
red  light,  but  nearly  black  in  green  or  blue  light,  since 
the  cells  of  the  petals  are  filled  with  a substance  which 
absorbs  the  green  and  blue  light.  On  the  other  hand, 
a red  rose  or  a carnation  will  appear  a brilliant  blue  in 


i8 


PHOTOGRAPHY  IN  COLOURS 


blue  or  greenish-blue  light,  or  if  seen  through  spectrum- 
blue  glasses,  but  appears  unaltered  in  a red  or  white 
light. 

If  a dilute  coloured  fluid  be  placed  in  a white  basin, 
the  white  light  which  penetrates  the  fluid  will  be  re- 
flected from  the  interior  of  the  basin,  and  after  being 
deprived  of  certain  constituents  will  appear  of  its  proper 
colour  to  the  eye.  If  the  same  fluid  be  placed  in  a 
black  basin  all  the  light  will  be  absorbed,  and  the 
liquid  will  appear  black.  If,  however,  some  white 
powder  be  sprinkled  into  the  basin,  the  white  particles 
will  reflect  and  scatter  the  light  so  that  the  original 
colour  is  restored  to  the  liquid. 

The  colour  of  pigments  is  entirely  due  to  their  power 
of  absorption.  The  entering  light  penetrates  a slight 
distance  into  and  between  the  particles,  and  is  reflected 
back  after  losing  all  those  colours  which  do  not  con- 
tribute to  its  proper  colour.  Thus  a mixture  of  Prussian 
Blue  and  Gamboge  appears  green  to  the  eye  because 
the  blue  paint  absorbs  all  the  red,  orange,  and  yellow 
rays,  and  transmits  the  green  and  blue;  while  the 
yellow  paint  absorbs  the  violet,  indigo,  and  blue  rays. 
Hence  the  green  rays  are  the  only  ones  left  to  be 
reflected  by  both,  and  therefore  the  mixture  appears 
green. 

§ 10.  Surface  Colours. — Some  bodies  possess  the 
peculiar  property  of  reflecting  one  colour  and  transmit- 
ting another.  Thus  if  an  alcoholic  solution  of  Aniline 
be  allowed  to  evaporate  on  a glass  slide,  it  will  appear  of 
one  colour  when  seen  by  reflected  light,  and  its  com- 
plementary colour  when  seen  by  transmitted  light.  It 
is  well  known  that  gold-leaf  transmits  green  light,  and 


THE  NATURE  OF  LIGHT  AND  COLOUR  1 9 

reflects  yellow  rays.  Cyanine,  again,  is  purple  by 
transmitted  light,  but  of  a peculiar  plum  colour  by 
reflected  light.  This  is  due  to  a very  dark  absorption 
band  in  the  green  which  absorbs  this  colour,  and  hence 
only  the  blue  and  red  rays  reach  the  eye. 

§ 11.  Saturation. — A colour  is  said  to  be  saturated 
when  it  is  not  combined  with  white.  If  a beam  of  white 
light  is  made  to  pass  through  two  coloured  glasses  of 
the  complementary  colours  they  appear  black,  since 
each  absorbs  all  the  colours  except  the  one  it  transmits. 
Thus  a pure  red  glass  only  lets  red  light  through,  and 
a pure  green  glass  only  transmits  green  light.  Hence 
no  light  at  all  passes,  and  therefore  when  superposed 
they  appear  black  when  held  up  to  the  light.  By  this 
means  a strong  light  may  be  reduced  to  any  desired 
degree.  In  the  same  way,  if  a body  of  any  colour  be 
viewed  through  a coloured  glass  which  absorbs  the 
rays  reflected  by  that  body,  the  latter  appears  black. 
For  example,  a green  body  appears  black  when  viewed 
through  a red  glass  of  the  proper  shade,  since  the  green 
rays  which  are  reflected  are  all  absorbed  by  the  red 
glass.  On  the  other  hand,  a body  viewed  through  a 
glass  of  the  same  colour  appears  of  the  same  tint 
and  hue  as  a sheet  of  white  paper  placed  beside  it. 
Hence  the  apparent  colour  of  a body  varies  with  the 
light  by  which  it  is  viewed.  Thus  blues  and  greens 
are  nearly  indistinguishable  in  a yellow  artificial  light, 
such  as  a candle  or  gas  light. 

Yellow,  as  we  shall  prove  later  (p.  46),  is  not  a primary 
colour,  and  it  is  formed  in  the  spectrum  by  the  overlap- 
ping of  the  red  and  green  ; but  in  the  eye  it  appears  to 
be  a distinct  sensation,  since  if  one  blinds  one’s  eyes 


20 


PHOTOGRAPHY  IN  COLOURS 


temporarily  by  gazing  for  a long  time  at  a bright  Sodium 
flame  through  a spectroscope,  one  will  observe  that 
the  red  and  green  run  into  each  other  with  no  trace 
of  yellow.  This  shows  a remarkable  fact,  viz.  that 
with  ourselves  yellow  is  a distinct  colour,  and  not 
merely  a mixture  of  red  and  green,  such  as  is  evolved 
by  the  fusion  of  those  colours  in  Helmholtz’  experi- 
ments. Furthermore,  yellow  light  does  not  fatigue 
the  eye  for  either  red  or  green. 

§ 12.  Transparency  and  Translucency. — A body  is 
said  to  be  transparent  when  light  passes  freely  through 
it  with  a minimum  of  absorption  or  reflection.  It  is 
said  to  be  translucent  when  it  only  transmits  a portion 
of  the  light,  or  scatters  some  of  it.  Thus  frosted  glass, 
horn,  and  tortoise-shell  are  said  to  be  translucent. 
Some  of  the  light  is  transmitted,  but  much  is  absorbed 
or  scattered,  so  that  objects  are  seen  dimly  through 
them.  The  substance  through  which  the  light  passes, 
whether  it  be  gaseous,  liquid,  or  solid,  is  termed  the 
Medium.  If  it  be  uniform  in  all  respects,  it  is  termed 
homogeneous  ; if  not,  it  is  called  heterogeneous. 

Transparency  and  translucency  are  only  relative 
terms,  since  no  substance  is  absolutely  transparent  or 
absolutely  opaque.  Below  sixty  fathoms  the  sea  would 
appear  pitch  dark  to  the  eye.  Opaque  substances,  if 
sufficiently  thin,  will  permit  of  some  light  passing 
through.  Thus  gold-leaf  will  transmit  green  light  when 
held  in  front  of  a lamp,  and  direct  sunlight  will  pene- 
trate through  the  wood  of  any  dark  slide  in  sufficient 
quantity  to  fog  a photographic  plate  if  the  sun  be  allowed 
to  shine  on  the  slide  for  a quarter  of  an  hour,  or  even 
less.  Iron,  silver,  and  copper  in  very  thin  layers  will 


THE  NATURE  OF  LIGHT  AND  COLOUR  2 1 


transmit  some  trace  of  light.  An  opaque  body  may 
be  rendered  invisible,  and  sometimes  even  transparent, 
by  making  it  incapable  of  reflection.  Thus,  if  a drop  of 
Canada  Balsam  be  placed  on  the  ground-glass  surface 
of  a camera  focussing-screen  and  a microscope  cover- 
glass  be  gently  pressed  over  it,  the  inequalities  of  the 
surface  are  filled  up  by  the  balsam.  As  this  latter  is 
of  the  same  index  of  refraction  as  the  glass,  the  screen 
becomes  quite  transparent  and  indistinguishable  from 
a piece  of  clear  glass  at  that  spot.  By  this  means  the 
most  minute  details  can  be  readily  focussed-up  by  a 
magnifying  glass.  In  the  same  way  a piece  of  paper  can 
be  made  translucent  by  smearing  it  with  oil  or  grease. 
The  air  spaces  and  fibres  of  which  the  paper  is  composed 
have  a different  refraction  from  the  intervening  par- 
ticles, so  that,  when  the  whole  is  saturated  with  oil, 
the  indices  of  the  parts  are  nearly  equalised,  and,  the 
paper  being  rendered  homogeneous,  very  little  light  is 
scattered.  Again,  if  a glass  rod  be  placed  in  a test- 
tube,  the  rod  can  be  clearly  seen ; but  if  the  tube  be 
filled  with  Cedar  oil,  which  has  nearly  the  same  re- 
fractive index  as  the  rod,  the  part  of  the  rod  which  lies 
in  the  oil  becomes  at  once  invisible.  The  absence  of 
colour  becomes  of  the  highest  importance  in  the  case 
of  photographic  lenses.  Thus  a very  faint  tinge  of 
yellow  in  the  glass  which  is  quite  invisible  when  held 
up  to  the  light,  and  only  perceptible  when  placed  on  a 
sheet  of  white  paper,  will  increase  the  exposure  as 
much  as  20  per  cent,  or  30  per  cent,  owing  to  the 
absorption  of  the  blue-violet  and  ultra-violet  rays. 

§ 13.  Reflection  is  the  rebound  of  light-waves  from 
the  surface  on  which  they  are  incident,  into  the  original 


22 


PHOTOGRAPHY  IN  COLOURS 


medium.  If  the  surface  is  polished  and  uniform  the 
reflection  is  said  to  be  regular,  and  the  image  of  the 
source  of  light  can  be  seen.  If  roughened  or  uneven 
the  reflection  is  irregular,  in  which  case  the  surface  can 
be  seen,  but  the  image  of  the  source  cannot  be  seen. 
Mercury  and  polished  silver  reflect  about  90  per  cent,  of 
the  light,  and  a silvered  mirror  about  80  per  cent. 
Fresh  fallen  snow  is  a good  example  of  irregular 
reflection  or  scattering  of  light  in  which  nearly  all  the 
light  is  reflected. 

§ 14.  Shadows. — When  an  opaque  body  interrupts  a 
portion  of  the  light  within  an  illuminated  area,  it  casts 
a shadow  on  the  surface  in  front  of  it.  A region  which 
is  screened  off  from  a radiation  or  vibration  of  any  kind 
is  also  termed  a shadow,  although  the  latter  may  be  in- 
visible to  the  eye.  Thus  we  talk  of  “ sound-shadows  ” 
and  “ electric-shadows  ” whenever  an  obstruction  cuts 
off,  or  even  diminishes,  a series  of  vibrations.  This 
may  be  observed  the  moment  when  one  suddenly  turns 
a corner  from  a noisy  to  a quiet  street.  The  sound  of 
the  traffic  in  the  street  becomes  at  once  lessened.  One 
is  in  a “ sound-shadow.” 

A shadow  cannot  produce  an  image,  but  only  a 
silhouette  of  the  object  which  obstructs  the  light  from 
the  source.  In  nature  the  source  is  never  entirely  a 
point  of  light,  but  consists  of  a very  large  reflecting 
surface  which  causes  interposed  objects  to  cast  greatly 
extended  half-shadows.  These  produce  the  well-known 
half-tones  which  so  greatly  add  to  the  beauty  of  land- 
scapes. Usually  the  sources  are  multiple,  so  that  each 
half-shadow  overlaps  others,  and  thus  breaks  up  the 
tones  still  further,  so  that  when  a surface  is  illuminated 


THE  NATURE  OF  LIGHT  AND  COLOUR  23 


from  two  sources  the  shadow  becomes  split  up  into 
four  parts.  First,  two  distinct  penumbrae  (half- 
shadows), one  from  each  source,  then  a darker  combined 
penumbra,  and  lastly,  a dark  umbra  where  no  light 
reaches  it  is  seen.  These  facts  are  of  the  greatest  impor- 
tance to  artists,  and  deserve  careful  study.  A some- 
what similar  phenomenon  to  the  formation  of  shadows 
occurs  when  direct  sunlight  passes  through  a hole  an 
inch  or  so  in  diameter.  If  the  opening  be  square  or 
irregular  in  shape,  and  the  screen  be  perpendicular  to 
the  line  of  light  the  image  will  be  square  or  irregular 
and  sharply  defined — provided  the  screen  be  not  distant 
many  times  the  diameter  of  the  opening.  On  the 
screen  being  removed  further  back,  the  image  will 
become  larger,  the  margins  less  distinct,  and  the  corners 
rounded  off,  until  at  length  the  image  forms  an  im- 
perfectly defined  disc.  If  the  screen  is  tilted  to  the  axis 
of  the  rays  the  disc  becomes  oval.  This  explains  why 
the  images  made  by  the  irregular  spaces  between  the 
leaves  of  a tree  form  circular  patches  of  light  on  the 
ground  when  the  sun  is  vertical,  and  become  more  and 
more  elliptical  as  the  sun  descends.  We  may  therefore 
say  that  when  the  screen  is  perpendicular  to  the  axis  of 
the  sunlight  through  the  aperture,  and  the  distance  of 
the  screen  not  many  times  greater  than  the  size  of  the 
opening,  the  image  will  be  an  exact  copy  of  the 
aperture ; but  if  the  distance  of  the  screen  be  large 
compared  with  the  aperture,  then  the  image  becomes 
circular  or  elliptical,  according  to  the  angle  at  which 
the  screen  is  placed. 

In  photography  a knowledge  of  these  facts  plays  a 
very  important  part.  If  one  takes  a group  or  portrait 


24 


PHOTOGRAPHY  IN  COLOURS 


out-of-doors,  and  there  are  trees  in  the  immediate  back- 
ground, the  apertures  between  the  leaves  will  appear  of 
their  normal  irregular  shape  if  the  lens  be  focussed  on 
them.  But — as  is  always  the  case  when  a group  or 
figure  forms  the  principal  subject — the  lens  is  focussed 
on  the  group,  the  trees  will  be  more  or  less  out  of  focus, 
and  in  consequence  the  apertures  between  the  leaves 
will  appear  as  round  white  discs  in  the  print  or  positive. 
The  larger  the  aperture  used,  the  bigger  will  be  the 
discs,  and  consequently  they  will  become  very  obtrusive 
and  spoil  the  effect  of  the  picture.  To  avoid  this  it  is 
necessary  either  to  focus  for  a point  behind  the  group 
so  as  to  bring  the  trees  into  sharper  focus,  or  to  stop 
down  the  lens  to  a small  aperture  (which  must  never 
be  greater  than  F/16  in  any  case),  but  F/32  is  much 
to  be  preferred  and  will  generally  suffice  to  prevent  the 
discs  being  noticed  at  all. 

It  is  generally  stated  that  the  brighter  the  light  the 
blacker  will  be  the  shadow  cast.  This  is  not  strictly 
true,  because  if  the  light  be  very  bright,  such  as 
direct  sunlight  for  example,  the  greater  part  of  the  light 
is  reflected  from  the  sky  and  surrounding  objects,  and 
some  of  this  will  be  thrown  on  to  the  shadow  and 
adjacent  parts,  thus  greatly  reducing  the  contrast. 
The  same  thing  happens  when  a photographic  plate  is 
over-exposed  or  fogged  by  light  getting  into  the  camera. 
The  developed  image  will  appear  flat,  and  will  give  a 
dull  picture.  Now  the  light  of  the  full  moon  is  only 

the  eooVoo  Part  of  of  sun  at  noon>  so 
reflected  light  is  very  feeble,  and  has  very  little  effect  on 
the  shadow  cast  on  objects  by  the  moon.  Hence  the 
contrast  between  the  shadow  and  the  surrounding 


THE  NATURE  OF  LIGHT  AND  COLOUR  25 

surface  is  very  marked,  and  the  shadow  appears  inky 
black,  which  is  never  the  case  with  a sun-shadow  out- 
of-doors. 

§ 15.  Coloured  Shadows.— If  a sheet  of  whitepaper 
be  pinned  against  the  wall  and  illuminated  by  a light 
held  a foot  or  so  away,  and  a slip  of  red  glass  be  held 
in  front  so  as  to  throw  a red  image  on  the  paper,  the 
shadow  cast  by  any  opaque  object  (such  as  one’s 
finger)  placed  between  the  red  glass  and  the  paper  at 
the  margin  of  the  red  light,  will  appear  of  the  comple- 
mentary colour,  viz.  green.  In  the  same  way  a yellow 
glass  will  cause  a blue  shadow.  Again,  if  in  the  twilight 
the  light  from  a blue  sky  is  allowed  to  fall  into  a room 
illuminated  by  a candle,  and  one  holds  an  opaque  rod 
in  front  of  the  latter  so  as  to  cast  a shadow  on  a white 
surface,  while  another  rod  be  held  so  as  to  cast  a 
shadow  from  the  blue  sky  on  to  the  same  paper,  the 
first  shadow  will  appear  blue,  and  the  second  yellow, 
while  the  approximation  of  the  two  shadows  will  only 
intensify  the  contrast. 

§ 16.  Shadows  cast  by  Lenses. — If  a concave  lens 
be  held  between  a light  and  a screen  in  a darkened 
room,  it  will  cast  a dark  central  shadow  just  as  if  it  were 
an  opaque  body.  The  transmitted  rays  being  divergent 
they  will  be  turned  away  from  the  spot  around  the  axis 
of  the  lens  and  will  be  concentrated  around  the  margin 
of  the  cone,  thus  forming  a highly  illuminated  ring. 
The  further  the  lens  is  removed  from  the  screen,  the 
larger  and  fainter  will  be  the  ring  of  light.  A convex 
lens,  on  the  other  hand,  if  held  at  its  focal  distance 
from  the  screen  will  throw  a bright  central  image 
(focus)  om  it,  which  is  surrounded  by  a dark  shadow, 


26 


PHOTOGRAPHY  IN  COLOURS 


since  the  rays  are  refracted  towards  the  axis  of  the 
lens.  Very  little  light  therefore  will  reach  any 
portion  of  the  screen  behind  the  lens,  except  around 
the  centre. 

A prism  acts  much  in  the  same  way  as  a convex 
lens.  It  throws  a dark  shadow  behind  it,  since  the 
light  is  deviated  towards  the  base  of  the  prism,  outside 
which  it  forms  a bright  patch,  which  can  be  made  to 
travel  right  away  from  the  shadow  if  the  prism  be 
withdrawn  further  from  the  screen. 

§ 17.  The  Colour  of  the  Sky. — In  the  case  of  ordin- 
ary obstacles  to  the  passage  of  light  their  size  is  enor- 
mously greater  than  the  reflected  waves  of  visible  light 
(which  vary  from  0,3  /x  to  0,7  /x,  a “ /x  ” being  the  3—0  Par^ 
of  a millimetre),  so  that  the  ordinary  laws  of  reflection 
bear  no  reference  to  wave-lengths  when  a wave  is  in- 
tercepted by  such  a large  obstacle.  The  screening 
action  is  perfect,  save  for  a small  diffraction-band 
around  its  edge.  Now  it  is  obvious  that  a small  obstacle 
will  not  be  so  effective  as  a screen  for  the  long  (red) 
waves,  as  for  the  shorter  (blue)  ones.  When  the 
source  of  light  is  low  down,  such  as  near  the  time  of 
sunset,  the  long  waves  will  be  more  freely  transmitted 
than  the  short  ones,  and  consequently  as  the  sun  sinks 
to  the  horizon,  the  white  light  from  it  will  pass  to 
yellow  and  then  into  orange  and  red.  In  the  same  way 
the  snow  on  a distant  mountain  at  sunset,  will  appear 
of  a reddish  tint.  But  as  the  long  waves  are  the  most 
freely  transmitted  the  short  ones  will  be  most  freely 
reflected  and  scattered.  Hence,  as  we  look  at  the  sky, 
the  light  gradually  becomes  bluer  towards  the  zenith, 
because  the  blue  rays  from  that  part  of  the  sky  are 


THE  NATURE  OF  LIGHT  AND  COLOUR  27 


reflected  and  scattered  in  all  directions.  This  is  the 
reason  why  the  light  from  the  sky  appears  blue,  unless 
it  is  intercepted  by  clouds  or  mists. 

In  the  same  way  we  can  explain  the  cause  of  the 
yellow  tinge  in  fogs,  especially  if  the  sources  of 
obstruction  are  due  to  large  particles  such  as  soot.  Of 
course  a great  deal  of  the  light  is  both  stopped  and 
modified  by  these  obstructive  particles  in  its  path  to 
our  eyes.  During  the  transmission,  the  rays  of  high 
refrangibility  ( i.e . the  blue-violet  rays)  are  stopped  or 
hindered,  while  those  of  low  refrangibility  {i.e.  the  red- 
orange  rays)  are  destroyed  by  scattering,  hence  the 
light  which  reaches  the  eye  must  be  weak  in  rays  from 
both  ends  of  the  spectrum.  In  other  words  the  light 
is  chiefly  made  up  of  blue  and  greenish  rays.  When 
the  particles  reach  a certain  size  they  will  affect  all 
waves  equally,  with  the  result  that  the  light  will  appear 
white.  In  the  low  atmosphere  the  obstructive  particles 
are  largely  due  to  dust  and  aqueous  vapour,  but  in  the 
high  atmosphere  they  are  probably  due  to  extremely 
minute  particles  of  matter  such  as  very  finely  divided 
vapour  or  ice  particles,  or  possibly  to  gas  molecules 
(Rayleigh).  The  blue  tints  of  a distant  mist,  the  smoko 
from  a cigar  or  a wood  fire  are  all  due  to  the  same 
scattering  of  the  light  by  exceedingly  minute  particles. 

Tyndall  formed  an  artificial  cloud  by  precipitating 
the  vapour  of  iodide  of  allyl  through  the  action  of  light. 
He  found  that  the  particles  gradually  increased  in  size, 
and  as  they  did  so,  the  blue  colour  disappeared  and  the 
scattered  light  appeared  white,  as  we  have  already 
stated  above.  If  at  this  stage,  the  light  which  has  now 
become  polarised,  be  observed  through  a nicol  prism 


28 


PHOTOGRAPHY  IN  COLOURS 


held  vertically,  viz.  in  the  position  in  which  ordinary 
scattered  light  would  be  extinguished,  the  blue  colour 
will  appear  again  much  richer  than  before.  This  deep 
blue  was  termed  by  Tyndall  the  “ residual  blue.”  Lord 
Eayleigh  confirmed  Tyndall’s  experiment,  using  for  his 
dust-cloud  a precipitate  of  sulphur  made  by  adding  a 
little  very  dilute  sulphuric  acid  to  a weak  solution  of 
hyposulphite  of  soda. 


CHAPTER  II 

§ 18.  The  Evolution  of  Colour  Photography 

Colour  photography  may  be  said  to  date  back  to  the 
time  of  the  Earbenlehre  of  Goethe  in  1810.  It  is  there 
stated  that  “if  a spectrum  produced  by  a prism  is 
thrown  on  to  moist  chloride  of  silver  paper,  if  the 
printing  be  continued  for  fifteen  minutes,  I observe  the 
following  : in  the  violet  the  chloride  is  a reddish-brown 
(sometimes  more  violet,  sometimes  more  blue),  and 
this  coloration  extends  well  beyond  the  limit  of  the 
violet ; in  the  blue,  the  chloride  takes  a clear  blue  tint 
which  fades  away,  becoming  lighter  in  the  green.  In 
the  yellow,  I usually  found  the  chloride  unaltered ; 
sometimes,  however,  it  had  a slight  yellow  tint.  In  the 
red,  or  beyond  the  red,  it  looks  a rose  or  lilac  tint. 
The  image  of  the  spectrum  shows  beyond  the  red  and 
the  violet,  a region  more  or  less  light  and  uncoloured. 
Beyond  the  brown  band  which  was  produced  in  the 
violet,  the  silver  chloride  was  coloured  a grey-violet  for 
a distance  of  several  inches.  In  proportion  as  the 
distance  from  the  violet  increased,  the  tint  became 
lighter.  Beyond  the  red,  on  the  contrary,  the  chloride 
took  a feeble  red  tint  for  a considerable  distance.” 

It  was  not  until  1868  that  any  satisfactory  explana- 
tion of  these  phenomena  was  forthcoming.  In  that 


PHOTOGRAPHY  IN  COLOURS 


30 

year  Zenker  showed  that  colours  could  be  produced  by 
stationary  light-waves  formed  in  the  layer  of  silver 
chloride.  It  was  further  found  that  many  body 
(pigment)  colours  were  highly  sensitive  to  light,  be- 
coming bleached  by  its  action.  Wiener  showed  that 
a light-sensitive  substance  can  only  be  bleached  by 
those  rays  which  are  absorbed  by  it,  all  other  rays 
being  either  transmitted  or  reflected.  In  fact,  a sub- 
stance is  called  red  because  it  reflects  red  rays  and 
absorbs  green  and  blue  ones,  and  so  for  other  colours. 
If,  therefore,  any  substance  happens  to  be  sensitive  to 
the  action  of  different  wave-lengths  of  light,  it  must  be 
due  to  rays  of  such  colours  as  will  be  absorbed  by  the 
body.  Quite  recently  Dr.  Smith  has  made  use  of  this 
principle  in  his  Uto-color  bleach-out  paper  (see  Chapter 
XI.),  and  Szczepanik,  Neuhaus,  Liesegang,  and  others 
are  still  working  in  the  same  direction. 

In  1890,  or  22  years  later,  Lippmann  confirmed 
Zenker’s  discovery,  and  succeeded  in  producing  inter- 
ference pictures  by  placing  a very  thin  chloride 
emulsion  plate  in  contact  with  a layer  of  mercury 
which  acted  as  a reflector.  By  this  means  the  light- 
waves from  the  object  which  pass  through  the  film 
meet  previous  waves  reflected  by  the  mercury  mirror, 
and  so  produce  the  interference  phenomena.  Thus  the 
silver  chloride  is  changed  at  the  spot  where  the  crest 
is  increased,  but  unaltered  at  the  spot  where  the  wave 
is  neutralised. 

Meanwhile  Vogel,  Obernetter,  Eder,  and  Abney  were 
experimenting  to  increase  the  range  of  the  collodion  plate 
affected  by  the  action  of  coloured  lights.  By  bathing 
a plate  in  Eosin,  Vogel  found  that  he  could  extend  the 


EVOLUTION  OF  COLOUR  PHOTOGRAPHY  3 1 


sensitivity  of  the  plate  from  the  yellow-green  as  far  as 
the  orange,  and  Abney,  going  a step  further,  extended 
it  to  the  red  by  means  of  Cyanin  blue.  This  was  soon 
applied  to  the  gelatine  plate,  and  so  orthochromatic 
plates  came  into  vogue.  Later  on  Aethyl  red  was 
employed  by  Miethe,  and  finally  Pinacyanol  was 
applied,  which  enabled  plate-makers  to  produce  a 
panchromatic  plate,  i.e.  one  sensitive  to  all  the  colours 
of  the  visible  spectrum,  or  roughly  speaking  corre- 
sponding to  wave-lengths  from  400  to  700  ^ (micro- 
millimetres). 

The  discovery  of  the  power  of  certain  dyes  to  render 
the  photographic  plate  sensitive  to  all  visible  colours 
was  the  one  step  needed  to  render  the  methods  of 
colour  photography  and  colour  printing  possible.  The 
early  Daguerreotype  plates  were  only  sensitive  to  the 
violet,  the  blue,  and  a little  of  the  green.  The  gelatine 
plates,  before  the  action  of  dyes  was  known,  were 
sensitive  to  the  violet,  blue-green,  and  a little  of  the 
yellow.  Eosin  and  Erythrosin  dyes  extended  their 
action  through  the  yellow  and  orange,  while  Aethyl 
red,  Cyanine  and  Pinacyanol  brought  the  sensitivity 
nearly  up  to  the  end  of  the  visible  red. 

Gradually  our  ideas  of  colour  were  built  up.  The 
early  suggestions  of  Thomas  Young  (1773-1829), 
elaborated  by  Helmholtz  in  Heidelberg,  established 
the  new  theory  which  has  been  called  the  Young- 
Helmholtz  theory  of  colour  vision.  Clerk  Maxwell  in 
Cambridge  further  exemplified  this  theory  and  showed 
that  every  possible  colour  and  shade  of  colour  was 
either  a blue-violet,  a green,  or  a red,  or  else  a mixture 
of  two  or  of  all  three  of  these  colours.  Ducos  du 


32 


P HO  TO  GRA  PH  Y IN  COLOURS 


Hauron  in  France,  and  Ives  in  America,  applied  these 
facts  in  practice  by  making  three  separate  negatives  of 
the  coloured  object  through  a red,  green,  and  blue 
glass  respectively.  Then  positives  were  made  and  the 
three  projected  on  to  a screen  through  the  same  glasses 
and  made  to  coincide.  Ives  went  further,  and  took 
stereoscopic  pictures  in  the  same  way  and  combined 
the  chromograms  by  means  of  his  Kromskop  instru- 
ment. In  this  way  the  picture  was  seen,  when 
brilliantly  illuminated  from  behind,  in  all  the  original 
colours  and  in  stereoscopic  relief.  Later,  relief  blocks 
were  made  on  bichromated  colloids  taken  through  the 
same  coloured  glasses,  and  impressions  made  by  means 
of  the  complementary  colours  either  from  inks  or  dyes, 
and  prints  made  one  over  the  other  on  the  same  sup- 
port, or  on  separate  thin  films  which  were  superposed. 
In  this  way  facsimiles  in  colour  were  produced  which 
could  be  viewed  by  reflected  light,  or  bound  up  as  trans- 
parencies to  be  seen  by  transmitted  light.  In  the  same 
way  all  the  beautiful  coloured  “ process  ” prints  are 
obtained.  This  fundamental  method  of  producing  prints 
has  been  elaborated  in  various  ways  by  Dr.  E.  Konig, 
and  by  Sanger-Shepherd;  also  by  the  Rotary  Company’s 
stripping  pigment  process,  the  Carbon,  Collotype  and 
Raydex  methods.  In  the  year  1904  Messrs.  Lumiere 
obtaineddirect  tran  sparencies  in  colour  by  means  of 
screens  coated  with  starch  grains  dyed  in  the  three 
primary  colours,  and  coated  with  a panchromatic  emul- 
sion. This  method  has  been  followed  by  others,  based 
on  the  same  principle  ; but  instead  of  covering  the  glass 
with  coloured  starch  grains,  the  makers  have  ruled  the 
glass  with  fine  lines  or  dots,  or  a combination  of  both, 


EVOLUTION  OF  COLOUR  PHOTOGRAPHY  33 


in  the  three  primary  colours.  Such  plates  yield  exceed- 
ingly brilliant  pictures  and  very  pure  and  intense  colours, 
but  they  do  not  possess  the  softness  or  range  of  hues 
and  tints  that  the  autochrome  plates  do,  nor  are  the 
colours  so  true  to  nature. 

At  length,  after  several  years  of  experimenting  and 
many  disappointing  failures,  bleaching-out  papers  have 
been  produced  by  Szczepanik,  and  especially  by  Dr. 
J.  H.  Smith,  by  which  copies  of  colour  positives  can 
be  printed  in  direct  sunlight,  which  may  be  fixed  and 
mounted  like  ordinary  photographic  prints.  Such 
copies  present  the  great  advantage  of  being  viewed 
by  reflected  light,  which  permits  of  their  being  hung 
on  the  wall,  or  pasted  into  an  album.  It  cannot  be 
denied  that  these  pictures  are  far  from  perfect,  the 
colours  are  impure  and  the  whites  never  come  out  as 
pure  whites,  but  the  process  is  being  improved  every 
day,  and  we  have  no  reason  to  doubt  that  in  a com- 
paratively short  time  such  pictures  will  take  a promi- 
nent position  on  the  walls  of  our  photographic  ex- 
hibitions. 

Finally  we  must  mention  the  achievement  of  Messrs. 
Urban  and  Smith,  and  later  by  Gaumont,  by  which 
kinematograph  pictures  in  colour  can  be  projected  on 
to  the  screen,  whereby  the  illusion  of  objects  in  motion 
is  greatly  increased.  This  method  will  be  found 
described  in  detail  in  Chapter  XII. 


D 


CHAPTER  Iir 


§ 19.  The  Eye  compared  with  a Camera,  and  the 
Retina  with  a Colour  Plate 

The  human  eye  is  a spherical  ball  almost  exactly  one 
inch  in  diameter,  closely  resembling  a plum,  the  stalk 
of  which  represents  the  optic  nerve.  This  latter  is  a 
round  hard  cord  measuring  J of  an  inch  in  diameter, 
made  up  of  an  immense  number  of  bundles  of  nerve 
fibres,  surrounded  by  a tough  sheath.  If  traced  back- 
wards it  will  be  found  to  pass  through  a hole  in  the 
back  of  the  orbit  where  it  enters  the  skull,  and  imme- 
diately to  cross  obliquely  towards  the  middle  line, 
where  it  becomes  intimately  united  with  the  nerve  of 
the  opposite  side.  Here  it  again  separates,  the  halves  of 
each  nerve  fusing  together  to  form  a flat  band.  This 
band  passes  along  the  under  surface  of  the  brain,  into 
which  it  soon  enters,  and  becomes  lost  in  the  great 
optic  nerve  ganglia  which  are  situated  in  its  substance. 
There  the  fibres  become  intimately  connected  with  the 
brain- cells,  which  interpret  the  visual  impulses  pro- 
jected from  the  eyeball,  and  enable  the  person  to 
perceive  mentally  the  images  formed  at  the  back  of 
the  eye.  The  eyeball  consists  of  three  coats.  1st.  An 
outer  thick  tough  fibrous  coat,  the  Sclerotic,  which 
serves  to  keep  the  eye  in  shape,  and  protect  the  deli- 
cate coats  inside  it.  2nd.  A dark  layer,  made  up  almost 


THE  EYE  COMPARED  WITH  A CAMERA  35 

entirely  of  a network  of  blood-vessels,  which  serve  to 
nourish  the  parts  around.  And  3rdly  the  Eetina.  This 
is  a highly  complicated  nervous  layer  formed  by  the 
spreading  out  of  the  fibres  of  the  optic  nerve  over  the 
entire  surface  of  the  back  half  of  the  eye.  It  forms 


Fig.  2. — Semi-diagrammatic  vertical  section  of  retina  showing 
the  layers  and  systems. 

the  receptive  layer  on  which  the  images  of  external 
objects  are  perpetually  being  formed.  Each  individual 
fibre  is  connected  with  a large  cell  called  a ganglion- 
cell, from  which  one  or  more  fibres  pass  directly  back- 
wards in  a very  complicated  manner.  These  ultimate 
nerve  fibrils  divide  up  into  a number  of  fine  short 


36  PHOTOGRAPHY  IN  COLOURS 


branches,  or  arborisations  as  they  are  called,  which 
are  connected  with  other  branches  from  a second  set 
of  nerve  fibrils,  and  which  terminate  in  a series  of 
peculiar  bodies  called  rods  and  cones.  These  form 
the  terminals  of  the  retina.  Seen  from  above,  they 


Fig.  3. — Cross  section  of  the  rods  and  cones  in  the  retina  of  a 
human  eye,  taken  midway  between  the  periphery  of  the  retina 
and  the  yellow  spot.  The  small  discs  are  the  rods,  the  large 
ones  the  cones.  Observe  that  the  dots  in  the  centre  of  each 
circle  (which  represents  a cross  section  of  a rod  or  cone)  are 
kept  in  the  middle  of  the  surrounding  insulating  substance 
by  radiating  fibres.  Each  dot  is  the  cross  section  of  an  axis 
cylinder  (terminal  nerve  fibril).  From  a photograph  of  a 
microscopic  section  prepared  by  the  Author. 

Fig.  4. — Represents  a magnified  fragment  of  a Lumiere  screen. 
The  black  discs  represent  red  grains,  the  shaded  green- 
coloured  grains,  and  the  white  discs  blue-violet  grains. 

Fig.  5. — Cross  section  of  the  cones  at  the  Fovea  (centre  of  the 
yellow  spot) ; cones  surrounded  by  a single  layer  of  rods  are 
to  be  seen  towards  the  circumference.  From  a microscopic 
section  made  by  the  Author. 

form  a mosaic  pattern  which  is  made  up  of  large  discs 
surrounded  by  one,  two,  or  more  rows  of  small  discs. 
These  are  closely  packed  together,  and  are  so  numerous 
that  they  amount  to  about  5,000,000  in  all.  The  ends 
of  the  rods  and  cones  lie  in  contact  with  a dark  layer 
of  six-sided  cells,  which  are  filled  with  minute  granules, 


Fig.  3. 


Fig.  4. 


Fig.  5. 


THE  EYE  COMPARED  WITH  A CAMERA  2,7 


and  have  a network  filled  with  fine  oat-shaped  crystals 
which  finds  its  way  under  the  action  of  light  between 
the  ends  of  the  rods,  and  retracts  again  in  darkness. 
Their  function  seems  to  be  to  prevent  halation,  while 
the  function  of  the  six-sided  cells  is  to  secrete  the 
retinal  purple — a remarkable  chamois  or  purplish- 
coloured  substance  in  which  the  ends  of  ’the  rods  are 
bathed.  Its  use  is  not  known,  except  that  it  is  in 
some  way  connected  with  vision,  and  that  it  bleaches 
when  exposed  to  light. 

Exactly  in  the  centre  of  the  retina,  and  on  the  line 
of  the  visual  axis  at  the  posterior  pole  (back  of  the 
eye),  is  a dark  reddish  spot,  about  2 mm.  or  2,5 
mm.  in  diameter — the  macula.  This  is  the  area 
of  most  distinct  vision,  and  here  the  rods  become 
greatly  reduced  in  number,  until,  at  the  centre  where 
there  is  a tiny  pit,  there  are  no  rods  at  all,  but  only 
cones.  These  are  not  cones  in  the  ordinary  sense  of 
the  word,  such  as  are  to  be  found  over  every  other 
part  of  the  retina,  but  long  narrow  cylinders  closely 
packed  together,  so  as  to  get  as  many  as  possible  in  a 
small  space.  By  this  means  the  sensitivity  of  the  spot 
is  largely  increased,  since  the  image  of  an  object  will 
cover  far  more  cones  than  if  it  were  to  be  formed  any- 
where else  on  the  retina.  We  therefore  only  see  dis- 
tinctly over  the  macula  area  a roundish  patch  about 
two  mm.  across,  and  which  subtends  an  angle  of 
about  5 degrees  in  the  field  of  vision.  Critical  vision 
is  confined  to  the  pit  itself,  which  is  a much  smaller 
area  still ; in  fact,  it  occupies  in  the  field  of  vision  an 
angle  of  less  than  1 degree  (about  8").  So  that  it  may 
literally  be  said  that  we  read  with  our  maculse  only, 


PHOTOGRAPHY  IN  COLOURS 


38 

and  not  with  the  whole  of  our  retinae,  although  we  can 
get  a general  impression  of  objects  over  the  whole 
retinal  field.  As  a matter  of  fact  we  only  see  one 
object  at  a time,  but  by  revolving  the  eye  very  rapidly 
both  up  and  down  and  horizontally,  everything  can  be 
seen  satisfactorily.  It  is  to  our  advantage  that  we  do 
not  see  everything  sharply  at  the  same  moment,  other- 
wise we  should  see  such  an  overwhelming  mass  of 
detail,  that  we  should  be  quite  unable  to  take  the  view 
in,  or  pay  attention  to  anything.  As  it  is  we  see  one 
point  after  another  in  succession,  and  by  combining 
the  sum  of  all  the  points  looked  at,  we  receive  the 
impression  of  the  whole  view  sharply  defined.  In  all 
the  lower  mammals  below  the  monkeys  we  find  no  trace 
of  a macula,  but,  on  the  other  hand,  we  find  an  extended 
sensitive  area.  So  that  most  of  the  animals  do  not  see 
so  well  as  we  do  over  a small  area,  but  they  see  much 
better  over  a larger  one.  This  is  the  reason  why  no 
animal  (or,  more  strictly  speaking,  no  mammal)  below 
the  monkeys  habitually  moves  its  eye,  but  if  it  wishes 
to  look  at  an  object  in  another  direction  it  almost  in- 
variably turns  its  head,  but  not  its  eye.  Any  one  who 
takes  the  trouble  to  watch  the  eyes  of  any  animal  at 
any  Zoological  Gardens  can  prove  that  fact  for  him- 
self. 

If  you  look  at  Fig.  2 you  will  observe  that  each  cone 
has  a nerve  fibre  exclusively  belonging  to  it,  which  al- 
though twice  interrupted  in  its  course  by  arborisations, 
nevertheless  permits  the  current  to  pass  along  the  con- 
tinuation of  the  fibre  direct  to  the  sensorium.  Moreover, 
the  nerve  fibril  is  furnished  with  a relay  ganglion 
(e,  e , e , e,  Fig.  2).  The  rods,  on  the  other  hand,  have  not 


THE  EYE  CO  MPA  RED  WITH  A CAMERA  39 


each  a separate  nerve  which  connects  them  with  the 
brain.  If  you  examine  the  figure  carefully  you  will 
notice  that  from  2 to  10  or  more  rod  fibrils  are 
embraced  by  a single  arborisation,  so  that  the  square 
of  these  numbers  will  represent  the  number  of  rods 
which  have  united  to  convey  a single  impression  to  the 
brain.  Hence  100  or  more  rods  will  only  convey  the 
impression  of  the  same  amount  of  detail  that  a single 
cone  will  effect. 

It  is  the  cones,  therefore,  which  carry  the  image- 
forming vibrations  to  the  sensorium.  The  function  of 
the  rods  appears  to  be  twofold.  1st.  They  act  as 
dampers  to  the  acuity  of  vision  outside  the  macula 
area,  so  as  to  allow  the  mind  to  be  concentrated  on  the 
area  of  fixation.  2nd.  By  occupying  very  little  room, 
and  being  very  numerous,  the  rods,  although  they  do 
not  help  the  definition  of  objects,  enable  the  eye  to 
perceive  dimly  lighted  objects,  which  the  scattered 
cones  alone  would  only  perceive  very  imperfectly,  or 
not  at  all.  (See  the  Purkinje  Phenomenon,  § 25.) 

Occupying  the  whole  of  the  front  of  the  eye  is  a 
clear  transparent  modification  of  the  sclerotic  coat 
known  as  the  Cornea.  Inside  this  is  a coloured  mem- 
branous ring — the  Iris,  having  a black  circular  aperture 
in  the  centre— the  Pupil.  Behind  the  Iris,  and  resting 
against  it,  is  a highly  refractive  curved  body — the  Lens. 
Between  this  lens  and  the  cornea  is  a space  filled  with 
a clear  watery  fluid — the  Aqueous.  The  remainder  of 
the  eye  is  filled  with  a clear,  colourless,  thin  jelly 
known  as  the  Vitreous  Humour.  The  eye  thus  forms 
a spherical  camera  similar  in  many  respects  to  a pho- 
tographic camera.  In  the  eye  the  lens  system  is 


40 


PHOTOGRAPHY  IN  COLOURS 


formed  by  two  lenses : lstly,  the  cornea,  which  is  a 
spherically  curved  shell  of  dense  transparent  tissue ; 
and  2ndly,  the  crystalline  lens,  which  is  a strongly 
curved  biconvex  lens  placed  a little  distance  behind 
it,  and  separated  from  it  by  a watery  fluid — the 
Aqueous  (see  Fig.  6).  Between  the  two  lenses  lies  the 
Iris.  This  forms  the  variously  coloured  membrane 
which  contributes  so  largely  to  the  beauty  of  the  eye. 
It  forms  a perfect  iris  diaphragm,  having  a circular 
aperture  in  the  centre,  which  appears  black  to  the 
observer.  This  contracts  or  dilates  automatically  with 
the  increase  or  diminution  of  the  light.  At  its  full 
opening,  it  measures  from  6 to  8 mm.,  or  in  some 
people  even  10  mm.  across.  The  focal  length  of 
the  lens  combination  in  situ  is  15,5  mm.,  or  about 
3/5  in.,1  so  that  at  its  full  opening  it  works  at  F/2,5 
or  F/2,  and  in  some  cases  even  F/1,5,  and  in  a very 
bright  light  at  about  F/8.  This  fraction  is  understood 
to  mean  the  ratio-aperture  of  the  lens,  i.e.  the  ratio 
between  the  focal  length  of  the  lens  and  the  diameter 
of  the  stop  or  aperture.  Thus,  supposing  the  lens  to 
have  a focal  length  of  8 inches,  and  the  diameter  of 
the  stop  to  be  2 inches,  the  ratio  aperture  would  be  as 
8:2,  and  we  should  speak  of  the  lens  as  working  at 

1 This  is  the  case  if  we  measure  the  focal  length  from  the 
nodal  point  (situated  at  the  posterior  pole  of  the  crystalline  lens) 
to  the  retina.  If  we  take  the  focal  length  to  be  the  distance  from 
the  posterior  principal  plane  to  the  retina  (as  is  done  when  work- 
ing out  the  measurements  of  the  eye),  then  the  focal  length  will 
be  approximately  20  mm.  But  the  former  distance  is  the  most 
convenient  for  our  purpose,  since  the  distance  between  the  nodal 
point  and  the  retina  is  the  one  used  for  determining  the  magnify- 
ing power  of  the  eye  and  the  size  of  the  retinal  image. 


THE  EYE  COMPARED  WITH  A CAMERA  4 1 


P/4,  and  the  same  applies  to  every  other  focal  length 
and  stop. 

The  camera  lens  usually  consists  of  two  lenses  (or 
rather  of  two  cemented  combinations),  and  there  is 
also  an  iris  diaphragm  working  between  them.  The 
ratio -aperture  varies  from  F/3  in  a very  rapid  portrait 
lens,  or  P/8  the  full  aperture  of  a rapid  rectilinear 
lens,  to  P/45  its  smallest  aperture.  Hence  the  eye 
has  a great  advantage  over  all  manufactured  lenses  as 
regards  rapidity.  Moreover,  a camera  lens  rarely  em- 
braces an  angle  of  over  100°,  whereas  the  eye  embraces 
an  angle  of  160°,  or  about  170°  when  both  eyes  are 
open.  The  eye  camera  is  a rigid  one,  and  it  is  adjusted 
for  various  distances  by  altering  the  curve  of  the  front 
surface  of  the  lens.  This  is  effected  by  means  of  a 
circular  band  of  muscular  fibres  embedded  in  the  coats 
of  the  eye,  and  surrounding  the  edge  of  the  lens.  When 
this  circular  muscle  contracts,  it  relaxes  its  tension  on 
a ligament  which  presses  against  the  front  of  the  lens, 
thereby  allowing  it  to  bulge  forward,  and  thus  increasing 
its  refractive  power.  Hence  if  the  eye  were  previously 
focussed  for  infinity,  it  would  now  be  adapted  for 
nearer  and  nearer  objects  in  direct  proportion  to  the 
amount  of  contraction  of  the  ciliary  muscle,  and  con- 
sequently to  the  amount  of  bulging  forward  of  the  front 
surface  of  the  lens.  In  the  photographic  camera  the 
same  result  is  obtained  by  racking  the  lens  further 
away  from  the  screen. 

In  the  camera  the  rays  come  to  a focus,  and  thus 
form  a picture  on  the  ground-glass  screen,  or  on  the 
sensitive  plate  when  that  replaces  it.  In  the  same 
way  the  rays  are  brought  to  a focus  on  the  retina 


42 


PHOTOGRAPHY  IN  COLOURS 


(B,  Fig.  6)  at  the  back  of  the  eye  by  means  of  the  lens 
system,  the  image  falling  on  a fine  mosaic  consisting 


Fig.  6. — Horizontal  section  of  the  human  eye  through  the 
Macula  and  optic  nerve  : magnified  x 2. 

Cj.  Conjunctiva  or  Membrane  which  spreads  over  the  eye  to  the 
rim  of  the  Orbit. 

Scl.  Sclerotic  or  fibrous  capsule  of  the  eye. 

Vi.  Vitreous  humour  (or  jelly)  filling  the  cavity  behind  the  lens. 

Ch.  Choroid  or  vascular  layer  which  secretes  the  visual  purple 
and  also  nourishment  for  the  rods  and  cones. 

R.  Retina,  or  expansion  of  the  optic  nerve  adapted  to  receive 
impressions  of  light.  This  is  a complex  nervous  structure, 
the  fibres  of  which  terminate  in  minute  rods  and  cone- 
shaped  bodies.  They  are  closely  packed  together,  forming 
a mosaic  when  viewed  on  the  surface. 

M.  Macula  (yellow  spot),  the  highly  sensitive  area  of  the  retina. 

F.  Fovea  or  centre  of  the  Macula. 

ON.  Optic  Nerve  (cut  short)  leading  to  the  brain. 

of  a layer  of  rod  and  cone  terminals.  Behind  this 
mosaic  is  a layer  of  dark  pigment  cells  which  secrete 


( 


Plate  II. 


Normal  Fundus  (background  of  the  eye)  of  a man 
forty  years  old. 

By  permission  of  Prof.  Dimmer , of  Gratz. 

The  oval  patch  on  the  left  is  the  disc  or  head  of  the  Optic 
Nerve.  A little  to  the  right  of  the  centre  is  a dark  round 
patch.  This  is  the  most  sensitive  part  of  the  eye,  and 
comprises  the  macula  (yellow  spot)  area.  In  the  centre  of 
this  patch  is  a depression  or  pit  known  as  the  fovea.  The 
fixation  point  or  region  of  perfect  vision  is  only  to  be  found 
at  the  fovea.  The  dark  lines  are  due  to  blood-vessels, 
arteries,  and  veins,  which  enter  the  eye  near  the  centre  of 
the  disc  and  are  distributed  over  the  greater  part  of  the 
retinal  background. 


To  face  p.  43. 


THE  EYE  COMPARED  WITH  A CAMERA  43 


the  visual  purple,  and  behind  these  cells  is  a dense 
layer  of  blood-vessels  held  together  by  tissue,  and 
known  as  the  choroid  (Ch).  The  analogy  between  the 
way  in  which  light  acts  on  the  retina  and  on  a Lipp- 
mann  film  is  very  close.  In  the  Lippmann  camera  the 
light  passes  through  the  lens  and  sensitive  chloride-of- 
silver  film,  and  is  reflected  from  the  mercury  mirror  in 
contact  with  it  behind.  In  the  case  of  the  eye  the  light 
passes  through  the  lens  and  retina  and  is  reflected 
from  the  concave  spherical  mirror  which  constitutes 
the  choroid,  directly  on  to  the  ends  of  the  rods  and 
cones,  thus  giving  rise  to  image-forming  vibrations 
which  pass  along  the  fibres  of  the  optic  nerve  (ON) 
to  the  brain,  where  the  psychic  transformation  into  a 
visual  image  takes  place.  It  is  conceivable  that  if 
reflected  rays  meet  the  entering  rays  half  a wave- 
length later  in  a different  phase,  interference  phenomena 
will  be  produced  in  the  eye. 

In  colour  photography  we  place  a colour  screen 
with  red,  green,  and  blue-violet  dots,  patches,  or  lines 
in  front  of  the  sensitive  film.  In  the  eyes  of  mammals 
there  are  screens1  with  zones  of  brilliant  colours — 
emerald  green,  gold,  vermilion,  scarlet,  ruby,  orange, 
yellow,  brown,  blue,  violet,  and  purple,  in  a similar 
way.  In  man  and  many  other  mammals,  birds,  and 
reptiles  we  find  a monochromatic  vermilion  or  scarlet 
colour- screen.  In  a few  animals  ( e.g . albinos)  we  find 
a creamy  white  fundus,2  interspersed  with  patches  of 

1 These  coloured  screens  are  situated  in  the  front  part  of  the 
choroid,  immediately  behind  the  retina,  and  in  some  animals 
constitute  what  is  known  as  the  Tapetum  Lucidum  (Brilliant 
layer). 

2 Or  background  of  the  eye  (see  Plate  II.). 


44 


PHOTOGRAPHY  IN  COLOURS 


vermilion,  which  reflects  all  colours,  and  in  a few  we 
find  a chocolate-coloured  screen,  but  in  no  animal  is 
there  a black  screen  to  be  found,  which  ought  to  be 
an  essential  requisite  if  the  usually  accepted  theory 
were  true  that  the  image  of  external  objects  is  pro- 
jected on  to  the  layer  of  rods  and  cones,  instead  of 
being  formed  on  the  mirror  and  reflected  back  again 
on  to  their  ends  as  we  have  stated. 

The  photographic  plate  is  coated  with  a thin  layer 
of  gelatine  and  rendered  sensitive  to  light  by  being 
impregnated  with  Bromiodide  of  Silver.  In  the  eye 
we  have  a mosaic  made  up  of  the  ends  of  the  retinal 
nerve  fibres  on  which  the  image  of  the  object  seen  is 
focussed,  and  which  is  rendered  more  sensitive  to  light 
by  the  presence  of  the  visual  purple  which  is  constantly 
being  secreted  and  in  which  these  nerve  terminals  are 
bathed. 

§ 20.  Reason  why  the  Yellow  Spot  is  Yellow. — In 

taking  a photograph  through  any  kind  of  colour-screen 
plate,  it  is  necessary  to  restrain  the  intense  action  of  the 
blue-violet  rays  by  placing  a yellow  colour-filter  in  the 
path  of  the  rays  in  front  of  the  screen.  In  our  own 
eyes,  and  in  those  of  all  other  vertebrates,  we  have  a 
yellow  colour-filter  interposed  for  the  same  purpose 
throughout  the  entire  extent  of  the  retina.  This  occurs 
in  the  narrow-meshed  plexus  (Ch)  of  the  capillary 
vessels  which  lies  immediately  in  front  of  the  sensitive 
layer.  The  only  exception  is  at  the  fovea,  i.e.  the  tiny  pit 
at  the  centre  of  the  macula  (M)  or  yellow  spot  as  it  is 
called.  Here  there  are  no  blood-vessels,  and  nature 
therefore  has  placed  at  this  spot  a yellow  pigment  behind 
the  sensitive  layer  which  serves  the  same  purpose  even 


PLATE  III. 


Fig.  I. 


A.  Appearance  of  the  coloured  oil  globules  in  the  retina  of  a tortoise’s  eye, 
as  they  would  appear  if  seen  from  above  looking  towards  the  choroid,  x 700. 
G.  L.  Johnson. 

B.  Do.  do.  Appearance  of  the  oil  globules  in  the  retina  of  a domestic  fowl, 
X 500.  G.  L.  Johnson. 

C.  Appearance  of  the  starch-grain  layer  in  an  Autochrome  plate,  x 140.  G.  L. 
Johnson. 


£;/..«  K.  % M A r:  « ' f - * • 

' * a*  * gf*<  MB'  M 


life 


A ,, 


Fig.  II. 

Vertical  section  of  a pigeon’s  retina,  showing  appearance  and  position  of  the 
coloured  oil  globules,  x 870.  (After  Prof.  v.  Greeffs  monograph  in  the  Graefe- 
Saemisch  Handbuch  der  Gesamten  Augenlik.) 


To  face  p.  45. 


THE  EYE  COMPARED  WITH  A CAMERA  45 


more  effectively.1  If  there  were  no  yellow  pigment  at 
the  macula,  when  looking  at  a white  sky  or  white 
surface  we  should  see  a blue-violet  disc  projected  in 
the  line  of  regard  in  front  of  our  eyes,  corresponding 
to  the  n on-vascular  area  in  the  centre  of  the  visual  pit.2 

The  amount  of  blue-violet  absorbed  by  the  capillary 
screen  is  very  considerable,  extending  from  G to  K 
(4300  to  3920),  and  resembling  in  its  colour  and 
action  that  of  a Lumiere  screen. 

§ 21.  Remarkable  Similarity  between  the  Auto- 
chrome Colour  Screen  and  the  Colour  Screen 
in  certain  Birds  and  Reptiles. — If  we  examine 
a Lumiere  Autochrome  or  a Thames  colour  plate 
with  a magnifying  glass,  after  having  stripped  off 
a piece  of  the  sensitive  film,  we  notice  the  whole 
surface  is  covered  with  a mosaic  of  red,  green,  and 
blue- violet  dots.  Now  all  the  birds  and  most  of  the 
reptiles  show  no  trace  of  vessels  which  nourish  the 
retina,  which  we  find  so  highly  developed  in  most  of 
the  mammals.  In  the  majority  of  the  mammals  we 
find  the  retina  nourished  by  a rich  supply  of  blood- 
vessels and  capillaries,  but  in  all  the  birds  and  most  of 
the  reptiles  this  blood-supply  is  completely  wanting, 
the  nourishment  being  derived  by  osmosis,  or,  in  other 
words,  by  a soaking- through  or  percolation  of  choroidal 
lymph-plasma  through  the  layers  of  the  retina.  Con- 
sequently they  have  neither  a brightly  coloured  fundus 
nor  any  tapetum  lucidum  ( i.e . a reflecting  colour-layer)  of 

1 The  Author  ventures  to  offer  this  theory  as  the  true  explana- 
tion why  the  fovea  is  yellow.  As  far  as  he  knows,  it  is  here 
advanced  for  the  first  time. 

2 See  the  Author’s  work,  “ Pocket  Atlas  and  Test-book  of  the 
Fundus  Oculi,”  Adland  & Son,  London,  1910. 


46  PHOTOGRAPHY  IN  COLOURS 


any  kind  such  as  we  find  in  most  of  the  mammals  ; but 
instead,  some  of  them  are  provided  with  a mosaic  of 
oil  globules.  These  latter  are  tiny  droplets  of  a highly 
refracting  coloured  substance,  and  they  are  situated 
just  where  the  cones  penetrate  the  external  limitating 
membrane,  and  consequently  immediately  in  front  of 
the  sensitive  layer  ( i.e . tips  of  the  cones)  where  they 
receive  the  light  impression.  Their  position  will  be 
seen  to  be  almost  in  contact  with  the  pigment-laden 
filaments  which  pass  between  the  separate  nerve  ends. 
See  Plate  III. 

It  will  also  be  noticed  that  these  coloured  drops  form 
four  rows.  First  a yellow,  then  a red,  then  another 
yellow,  and  lastly,  a green  row.  In  the  tortoises  we 
find  in  addition  a blue-violet  row.  Thus  Lumiere’s 
discovery  has  been  forestalled  by  the  reptiles  and  the 
birds.  But  why  should  there  be  a yellow  layer  of  dots  ? 
Are  not  the  three  primary  colours  enough  ? In 
coloured  light  mixing  by  spectroscopic  colours,  Clerk 
Maxwell,  Helmholtz,  and  Abney  successively  showed 
that  the  three  primary  colours,  red,  green,  and  blue- 
violet  were  sufficient  to  produce  all  the  different  shades 
of  colour  ; and  in  colour  printing  likewise  every  colour 
could  be  produced  by  these  three  primaries  if  only  a 
shade  of  grey  were  added,  as  Mr.  Handel  Lucas  has  so 
ably  pointed  out  (see  “ British  Journal  of  Phot.,”  supple- 
ment, 1912,  pages  43  and  55) ; although  I agree  with 
Messrs.  Newton  and  Lucas  that  in  this  latter  case  the 
use  of  grey  is  only  necessary  on  account  of  the  imper- 
fection of  our  printing  inks.  But  the  perception  of 
colour  by  the  eye  differs  in  one  respect  from  the  purely 
physical  production  of  colour.  In  the  latter  case 


THE  EYE  COMPARED  WITH  A CAMERA  47 


yellow  is  produced  by  the  mixture  of  green  and  red  in 
certain  proportions.  In  the  eye,  however,  yellow  is  a 
distinct  colour  sensation.  This  fact  has  been  made  use 
of  to  make  a special  form  of  test  for  colour  blindness, 
by  which  a pure  yellow  comprising  the  D line  is  trans- 
mitted through  the  slit  of  a spectroscope,  and  a second 
yellow  is  made  to  match  by  employing  two  other  slits, 
one  transmitting  a red  beam,  and  the  other  a green 
beam.  The  candidate  is  then  required  to  adjust  the 
green  slit  until  by  overlapping,  the  red  together  with 
the  green  forms  a perfect  match  with  the  uncombined 
yellow.  If  the  observer  were  colour-blind  to  red  or 
green,  the  match  could  not  be  made.  The  writer  has 
repeatedly  suggested  that  a screen-plate  of  four  colours 
would  give  a more  perfect  rendering  of  nature  than 
the  usual  three  colours.1  Perhaps  some  colour-plate 
makers  will  carry  out  the  experiment. 

§ 22.  Colour  Vision  and  Colour  Blindness. — In 
order  to  understand  the  rationale  of  three-colour  photo- 
graphy, it  may  be  useful  to  some  of  our  readers  if  we  try  to 
explain  the  nature  of  colour  vision  and  colour  blindness. 

The  light  impression  gives  rise  to  three  sensations 
which  are  quite  distinct — a light  sense,  a form  sense, 
and  a colour  sense. 

The  first  is  the  faculty  of  distinguishing  illumination 
and  its  degrees  of  intensity.  This  is  effected  in  the 
most  simple  case  by  the  presence  of  pigment  spots  in 
the  cuticle  of  an  animal  or  plant,  and  forms  the  most 
rudimentary  of  all  forms  of  eyes. 

1 According  to  Hering’s  theory  of  colour  visions  these  four 
colour  sensations  must  be  present  together  with  white  and  black, 
thus  forming  three  opposing  groups,  viz.  red  and  green,  yellow 
and  blue,  and  white  and  black  (see  Appendix,  p.  261). 


48  PHOTOGRAPHY  IN  COLOURS 


The  form  sense  is  a higher  development  of  the  sight 
faculty,  and  needs  a transparent  refracting  body  to 
form  a real  image,  and  nerve  terminals  to  convey  the 
collected  impression  to  the  animal’s  brain.  This  image 
may  be  quite  independent  of  colour. 

The  colour  sense  constitutes  a still  further  develop- 
ment of  vision,  which  we  will  now  discuss. 

As  was  first  shown  by  Newton,  white  sunlight  can  be 
resolved,  by  means  of  a prism,  into  six  distinct  colours, 
viz.  violet,  blue,  green,  yellow,  orange,  and  red.  These 
are  the  only  pure  spectrum  colours  which  most  of  us 
can  perceive,  although  some  people  can  see  indigo  as  a 
distinct  colour,  thus  making  seven  colours  in  all.  We 
have  reason  to  believe  some  animals  can  see  other 
colours  beyond  the  range  of  this  spectrum.  We  have 
stated  that  all  the  colours  in  nature  can  be  formed  by 
suitable  admixtures  of  blue-violet,  green  and  red,  while 
white  is  formed  by  the  action  of  these  three  colours 
together.  Black  is  not  a colour  at  all,  but  is  caused  by 
the  absence  of  all  colour  sensation.1  Thus,  if  these 
three  primary  spectrum  colours  be  projected  on  to  a 
screen  by  separate  lanterns  and  then  superposed,  the 
result  is  a white  disc  of  light,  the  colours  being  added 
together  (additive  method).  If,  now,  you  put  a red  glass 
in  front  of  a lantern  emitting  white  light,  on  the  top  of 
that  a blue,  and  finally  a green  glass,  since  each  glass 
absorbs  all  the  colours  except  its  own,  no  light  at  all 
will  reach  the  screen,  and  the  result  will  be  a black 
patch,  the  colours  being  subtracted  (subtractive  method). 
But  three  coloured  lights  are  not  really  necessary  to 
produce  black,  since  any  two  complementary  coloured 
1 See  end  of  this  Chapter. 


THE  EYE  COMPARED  WITH  A CAMERA  49 


lights  or  glasses  if  superposed  will  effect  the  same 
purpose  by  subtraction,  that  is  to  say,  each  colour 
will  absorb  (subtract)  all  the  colours  of  the  spectrum 
except  its  own,  and  in  the  same  way  any  two  com- 
plementary coloured  lights  if  mixed  will  produce  white 
(by  addition),  as  can  be  proved  by  the  reader  for 
himself. 

If  you  look  steadily  for  a minute  at  the  red  part 
of  the  spectrum  in  a spectroscope  illuminated  by 
a very  intense  light,  in  which  the  rest  of  the 
spectrum  is  cut  off,  and  then  look  at  the  entire 
spectrum  through  another  spectroscope  moderately 
illuminated,  you  will  see  the  blue-violet  and  green 
bands ; but  the  green  runs  right  into  the  yellow 
as  far  as  the  G line,  where  it  suddenly  ends.  The 
yellow  will  be  found  to  have  entirely  vanished — it  is  all 
green.  Now  rest  your  eyes  for  about  ten  minutes  and 
repeat  the  experiment  with  the  green  part  of  the  spec- 
trum, and  you  will  notice  that  you  can  again  see  three 
colours,  but  they  are  changed  to  violet,  blue,  and  red. 
The  green  has  quite  gone  and  the  blue  runs  straight 
into  the  red.  In  the  same  way  you  can  blind  your  eyes 
to  blue,  and  the  green  and  violet  will  be  seen  to  run 
into  each  other.  Again,  you  may  blind  your  eyes  to 
the  violet.  This  is  more  difficult,  as  it  requires  a longer 
gaze  and  a very  intense  light.  The  blue  will  still  be 
visible,  but  it  ceases  abruptly  at  the  violet  end.  Lastly, 
if  you  blind  your  eyes  to  the  yellow  by  gazing  for  a long 
time  at  a bright  sodium  flame,  you  will  observe  that 
the  red  and  green  will  run  into  one  another.  This 
forms  yet  another  proof  of  what  we  stated  in  the  pre- 
vious paragraph,  viz.  that  yellow  is  a distinct  colour 


50 


PHOTOGRAPHY  IN  COLOURS 


and  not  merely  a mixture  of  red  and  green,  as  is  evolved 
by  the  fusion  of  those  colours  in  Helmholtz’  experiments. 
Furthermore,  yellow  light  does  not  fatigue  the  eye  for 
either  red  or  green. 

To  sum  up.  A red-blind  person  sees  violet,  blue,  and 
green.  A green-blind  sees  red,  blue,  and  violet;  a 
violet-blue-blind  sees  a little  blue,  all  the  green,  and  a 
little  red,  but  no  yellow,  since  the  green  and  red  have 
met  together. 

Unfortunately,  it  has  been  impossible  up  to  the 
present  to  imitate  a pure  spectrum  violet,  or  to  find 
it  in  natural  colours.  According  to  Dr.  Edridge-Green, 
the  corn-flower  and  some  varieties  of  Lobelia  nearly 
approach  to  it,  so  that  we  have  to  content  ourselves 
with  a blue-violet  dye,  i.e.  a mixture  of  violet  and 
blue,  which  is  the  closest  imitation  of  violet  that  we 
can  procure.  It  is  quite  possible  that  some  dye  will 
yet  be  found  that  will  give  us  a pure  violet,  as  well  as 
a pure  blue.  The  other  colours  are  much  easier  to  find. 
Thus,  ruby  glass  and  the  purple  of  Cassius  (oxystannate 
of  gold)  form  fairly  pure  reds.  Sulphur  and  bichromate 
of  potash  make  good  yellows,  and  a saturated  solution 
of  ammoniacal  sulphate  of  copper  makes  a nearly  pure 
blue.  But  the  reader  must  not  go  away  with  the 
idea  that  these  primary  colours  stop  abruptly.  They 
each  run  on  far  beyond  the  point  at  which  they 
appear  to  stop  in  the  spectrum.  “ In  other  words,  the 
red  and  green  sensations  overlap,  as  do  the  blue  and 
green  and  also  the  violet  and  blue,  so  that  we  must 
take  the  middle  point  of  the  combined  overlapping  as 
the  natural  boundary  between  the  adjacent  sensations  ” 
(Burch). 


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THE  EYE  COMPARED  WITH  A CAMERA  51 


As  to  how  we  see  colours  we  are  quite  ignorant. 
Most  physiologists  assign  the  perception  of  colour  to 
the  cones,  leaving  to  the  rods  the  function  of  seeing 
feeble  luminosities.  Helmholtz’  theory,  that  certain 
cones  respond  to  the  stimulus  of  red  undulations,  others 
to  green,  and  others  to  blue- violet,  will  not  bear 
close  investigation.  Some  physiologists  assign  the 
sense  of  colour  as  well  as  perception  of  form  to  the 
action  of  the  visual  purple.  This,  again,  is  open  to 
objection  that  there  is  no  visual  purple  at  the  fovea, 
or  in  certain  animals,  e.g.  bats,  but  this  difficulty  is 
got  over  by  assuming  that  other  secretions,  such  as 
visual  white,  visual  green,  or  visual  yellow,  take  on 
the  same  function.  The  author  found  all  these  bodies 
in  the  retinae  of  animals.  Edridge-Green  has  sug- 
gested that  the  visual  purple  flows  into  the  fovea 
during  the  act  of  vision.  This  theory  explains  certain 
phenomena,  but  is  not  accepted  by  many  physiologists. 
Again,  we  have  reasons  for  believing  that  there  is  a 
colour  centre  in  the  brain.  A remarkable  case  bearing 
on  this  point  occurred  in  the  author’s  practice.  The 
patient  was  suffering  from  a form  of  creeping  paralysis 
which  gradually  affected  the  limbs  of  the  left  side.  At 
the  same  time,  as  more  and  more  of  the  muscles  became 
paralysed,  the  sense  of  colour  slowly  vanished  in  the 
corresponding  eye,  until  ultimately  the  patient  could 
see  no  colour  at  all,  everything  appearing  black,  grey, 
and  white,  like  an  engraving.  This  was  tested  by 
getting  the  patient,  who  was  a good  water-colour 
painter,  to  make  a coloured  drawing  of  the  spectrum, 
first  with  the  sound  eye  and  then  with  the  colour-blind 
one.  Notwithstanding  the  absence  of  all  sense  of 


52 


PHOTOGRAPHY  IN  COLOURS 


colour,  vision  was  hardly  affected  at  all,  and  the  colour 
sense  remained  perfect  in  the  right  eye,  while  that  of 
the  left  eye  never  returned  (see  Plate  IV.). 

Colour  blindness,  which  affects  about  four  out  of  every 
hundred  people  one  meets,  may  be  due  to  a deficiency  in 
light  perception  of  a portion  of  the  spectrum,  usually  in 
that  part  which  lies  at  the  red  end.  Dr.  Edridge- Green 
has  classified  colour-blind  people  according  to  whether 
they  can  perceive  six  colours,  five,  four,  three,  two, 
or  only  one  colour.  Thus,  one  who  distinguishes 
all  six  colours,  or  a hexachromatic  person,  may  be  con- 
sidered as  normal.  One  who  distinguishes  five  of  the 
spectrum  colours  confuses  orange-red  and  red,  orange- 
yellow  and  yellow,  rose-red  and  red,  purple-violet  and 
violet,  bluish-green  and  green.  A tetrachromatic  person 
sees  only  four  distinct  colours.  He  confuses  red,  orange 
and  rose-red,  greenish-blue  and  green,  and  pure  blue  and 
violet.  A trichromatic  person  sees  only  three  distinct 
colours.  He  confuses  red,  orange  and  rose,  most  of  the 
yellows,  and  blue,  violet,  and  purple.  He  also  confuses 
rose,  grey,  and  green,  and  many  browns,  as  well  as 
bluish-greens  and  greens.  A dichromatic  person  sees 
only  two  colours.  Thus,  red,  orange,  yellow,  and  green, 
all  seem  alike,  as  do  all  blues  and  violets.  He  also  con- 
fuses blue-green,  purple,  and  greys.  A monochromatic 
person  sees  no  colour  at  all.  Everything  appears  as 
impure  whites,  blacks,  or  greys.  One-  and  two-colour 
cases  are  exceedingly  rare. 

The  Young-Helmholtz  theory  fits  in  better  than  any 
other  with  the  phenomena  relating  to  colour  photo- 
graphy, but  it  by  no  means  harmonises  with  all  the 
facts  connected  with  colour  vision.  Thus,  a dichromatic 


THE  EYE  COMPARED  WITH  A CAMERA  53 


red-blind  person  ought  to  see  green  best,  whereas  he 
sees  yellow  most  distinctly.  Again,  the  phenomena  of 
after-images  cannot  be  explained  by  this  theory.  Nor 
does  it  account  for  the  additive  and  subtractive  forma- 
tion of  white  and  black  sensations  in  persons  possessing 
only  two  or  three  units  of  colour  perception.  Further- 
more, a pencil  of  coloured  light,  e.g.  light  which  has 
passed  through  a coloured  glass  focussed  on  a very 
minute  area  of  the  macula,  will  produce  the  sensation 
of  white,  whereas  it  ought  to  give  rise  to  a very  decided 
sensation  of  the  colour. 

Again,  as  we  have  already  pointed  out,  yellow  is  a 
distinct  colour  and  not  merely  a combination  of  red 
and  green  coloured  lights. 

Lastly,  if  it  were  true  that  the  retina  consisted 
entirely  of  three  groups  of  fibres,  corresponding  to 
red,  green,  and  blue-violet  sensations,  how  can 
persons  blind  to  all  these  colours  have  nearly  normal 
vision  ? and  how  can  they  see  white  objects  as  white  ? 1 

§ 22a.  The  Visual  Purple. — We  are  still  in  doubt  as 
to  the  function  of  the  substance  secreted  by  the  outer- 
most layer  of  the  retinal  (hexagonal  cell)  layer,  called  by 
Boll  the  “ visual  purple.”  According  to  some  writers, 
who  have  made  it  a special  study,  vision  depends  entirely 
on  its  decomposition.  The  author’s  theory,  which  he 
thinks  harmonises  best  with  the  facts,  is  briefly  as  follows : 
We  know  that  this  visual  purple  is  rapidly  decomposed 
in  the  presence  of  bright  daylight  and  at  the  same  time 
is  continually  being  re-formed.  Now,  in  bright  sunshine 
this  visual  purple  is  used  up  as  fast  as  it  is  secreted, 
so  that  if  one  steps  into  a dark  room,  the  purple  having 
1 See  Appendix,  p.  259,  “ Theories  of  Colour  Vision  ” 


54 


PHOTOGRAPHY  IN  COLOURS 


been  nearly  all  used  up,  one  cannot  see  anything,  and 
one  has  to  wait  a minute  or  two  until  the  purple 
accumulates,  which  it  quickly  does.  As  the  amount 
increases  the  vision  improves,  or,  to  use  a familiar 
expression,  the  eyes  get  accustomed  to  the  dim  light. 
If,  now,  one  steps  out  of  the  room  into  the  bright 
sunshine  the  amount  of  accumulated  purple  generates 
so  much  visual  energy  that  one  is  dazzled  and  almost 
blinded  for  the  moment  until  the  superfluous  store  of 
purple  is  decomposed.  It  may  be  objected  that  bats, 
which  can  certainly  see  in  a very  dim  light,  have  no 
visual  purple  at  all,  but  then  they  possess  a buff-grey 
visual  substance  which  answers  the  same  purpose. 

§ 23.  On  the  Meaning  of  the  Sensation  called 
Black. — We  have  stated  on  page  48  that  black  is  not 
a colour  at  all,  but  is  caused  by  the  absence  of  all 
colour  sensation.  This  requires  some  qualification. 
We  must  distinguish  between  the  absence  of  all  light 
stimulus  on  a portion  of  the  active  normal  retina,  capable 
of  conveying  a colour  sensation  to  the  brain,  and  a gap 
in  the  field  of  vision  produced  by  a part  of  the  back  of 
the  eye  not  adapted  for  conducting  a sensation  to  the 
brain. 

The  former  gives  rise  to  the  appearance  or  sensation 
of  black,  whereas  the  latter  does  not  give  rise  to  any 
sensation  whatever,  so  that  the  gap  is  not  perceived. 
For  example,  our  visual  field  is  not  bounded  by  black ; 
on  the  contrary,  it  fades  away  imperceptibly  into 
nothingness  in  all  directions.  Again,  the  head  of  the 
optic  nerve  or  papilla  (ON,  Fig.  6)  occupies  a space  in 
the  field  of  vision  about  the  same  size  as  that  of  the 
macula  area  (see  bottom  of  page  37).  It  is  known  as 


THE  EYE  COMPARED  WITH  A CAMERA  55 


the  blind  spot,  and  is  situated  a little  to  the  outer  side 
of  the  line  of  regard.  Although  light  of  every  colour 
reaches  this  spot  at  the  back  of  the  eye,  it  does  not 
appear  as  a black  or  white  disc  in  space,  since  we  are 
unconscious  of  any  defect  there.  If,  however,  we  shut 
the  Right  Eye,  and  hold  our  right  finger  about  a foot 
away,  exactly  in  the  line  of  vision,  and  then  place  our 
left  forefinger  close  to  it,  and  move  it  slowly  outwards 
to  the  left-hand  side  (without  moving  the  head  or  line 
of  regard),  the  top  of  the  finger  will  suddenly  disappear 
and  then  reappear  again.  This  is  due  to  the  gap  in 
the  field  of  vision  due  to  the  blind  spot.  Another 
simple  experiment  which  shows  the  same  thing  is  to 
draw  a black  cross  and  a dot  on  a sheet  of  paper, 
about  the  distance  between  the  pupils  apart  (61  mm. 
or  2§  in.  apart). 


Fig.  7. 


Close  the  right  eye  and  hold  the  paper  so  that  the 
round  spot  is  in  the  line  of  vision  of  the  left  eye,  about 
12  inches  away.  If  now  the  paper  is  slowly  with- 
drawn or  approached  towards  the  eye  a position  will 
be  found  at  which  the  cross  becomes  invisible.  On 
withdrawing  or  approaching  the  paper  from  this 
position  the  cross  will  again  become  visible.  By  this 
means  the  area  of  the  blind  spot  can  be  calculated  or 
mapped  out.  It  will  be  found  to  be  about  2-25  mm. 
in  diameter. 


CHAPTER  IY 


§ 24.  The  Sensitiveness  of  the  Photographic  Plate 
as  compared  with  the  Eye  to  different 
Parts  of  the  Spectrum 

In  order  to  demonstrate  the  effect  of  different  colours 
of  the  spectrum  on  a sensitive  plate,  we  may  draw  a 


Red.  Yellow.  Green.  Blue.  Violet. 


Fraunhofer  lines 
in  the  solar 
spectrum. 

Prism  dispersion. 


The  luminosity 
of  the  spectrum 
expressed  as  a 
print  from  a 
perfect  negative. 


Same  as  No.  2 
expressed  as  a 
curved  line. 


Fig.  8. 


curve  on  squared  paper  in  which  the  height  of  the  curve 
(ordinate)  represents  the  intensity  of  the  light,  while 


THE  SENSITIVENESS  OF  THE  PLATE  5 ? 


the  distance  it  is  projected  horizontally  (abscissa) 
represents  the  scale  of  wave-lengths  of  the  different 
colours  of  the  spectrum. 

In  Figs.  8 and  9,  a curve  is  shown  representing  the 
sensitivity  of  the  eye  to  different  parts  of  the  spectrum 
of  sunlight.  It  will  be  seen  that  the  curve  rises  rapidly 
from  the  red  towards  the  yellow,  and  slopes  very 
gradually  on  the  blue  side,  there  being  hardly  any 
intensity  at  the  blue  end. 


Red.  Orange.  Yellow.  Green.  Blue.  Violet.  Ultra-violet. 


700  650  600  550  500  450  400  350  300 


Fig.  9. — Luminosity  Curve  of  Eye  (dotted  line).  Ordinary  Non- 
colour Sensitive  Plate  (plain  line).  The  numbers  express 
the  wave-length  of  light  in  micrcmillimetres  {fi/j.).1 

Now,  when  a photograph  of  the  solar  spectrum  is 
taken  with  an  ordinary  plate,  we  shall  find  the  curve, 
which  is  altogether  wanting  at  the  red  end,  rises  to  a 
maximum  in  the  blue,  where  it  is  prolonged  far  beyond 
the  range  of  visibility  of  the  human  eye  (Fig.  9). 

By  bathing  the  plate  with  one  of  the  isocyanine 
dyes  (pinacynanol)  it  has  been  found  possible  to  extend 
the  curve  of  sensitivity  into  the  red  (Fig.  10),  and  pro- 
duce what  is  known  as  a panchromatic  plate,  which 

1 A micromillimetre  or  fifi  = l,000,000tb  part  of  a millimetre. 
Some  writers  express  the  wave-length  in  terms  of  Angstrom 
Unit  (A.U.),  which  is  the  ten-millionth  part  of  a millimetre. 
Thus,  500  fx./x  may  be  written  5000  A.U.  See  Appendix,  p.  285. 


58  PHOTOGRAPHY  IN  COLOURS 

must  be  used  for  all  methods  of  colour  photography 
(excepting  the  two-plate  method). 

A plate  dyed  with  eosin  or  erythrosin  (and  thereby 
rendered  more  sensitive  to  green  and  yellow  rays)  will 


JOO  650  600  550  500  650  600  550  500 

Fig.  10. — The  Spectrum  Curve  of  a Panchromatic  Plate. 


render  colours  sufficiently  true  to  nature  for  ordinary 
purposes.  Such  a plate  is  termed  iso-chromatic  or 
ortho- chromatic  (Fig.  11). 


Red.  Orange.  Yellow.  Green.  Blue.  Violet.  Ultra-violet. 


yOO  650  600  550  500  650  600  550  500 


Fig.  11. — The  Spectrum  of  an  Isochromatic  or  Orthochromatic 
Plate.1 

In  order  to  render  these  two  kinds  of  plates  more 
effective,  it  is  advisable  to  restrain  the  excessive  action 

1 As  it  is  rather  difficult  for  the  beginner  to  fix  in  his  mind 
the  numbers  which  correspond  to  the  various  colours,  Dr.  Mees, 
to  whom  photographers  owe  so  much  for  his  lucid  pamphlets  on 
this  subject,  has  suggested  an  admirable  mnemonic,  which  is  as 
follows : — 

The  wave-lengths  for  Blue-violet  lie  between  100  and  500  fifi 
,,  ,,  Green  ,,  500  and  600  /*/* 

,,  ,,  Red  „ 600  and  700  fj.fi 


THE  SENSITIVENESS  OF  THE  PLATE  59 


of  the  blue-violet  rays  by  the  use  of  a yellow  or 
green  colour-filter.  When  these  means  are  employed 
the  curve  of  sensitivity  of  the  plate  will  approximate 
more  nearly  to  that  of  the  human  eye. 

§ 25.  Purkinje  Phenomenon. — The  intensity  of  a 
coloured  light  is  largely  the  result  of  the  admixture  of 
mixed  (or  white)  light  with  it,  so  that  the  more  intense 
the  coloured  light,  the  whiter  the  colour  becomes  to  the 
eye ; and  conversely,  the  feebler  the  light,  the  more 
does  it  approach  black  in  shade.  But  this  latter 
change  is  by  no  means  equal  for  the  three  primary 
colours.  If  two  different  parts  of  the  spectrum,  say 
the  blue  and  the  red,  be  illuminated  so  that  the  bright- 
ness of  each  is  the  same,  and  then  the  common  light 
is  gradually  reduced,  the  brightness  of  the  two  colours 
will  no  longer  be  equal.  The  long  red  waves  become 
invisible  much  sooner  than  the  blue- violet  ones.  As 
the  light  gets  more  and  more  feeble  the  spectrum 
becomes  less  and  less  visible  towards  the  red  end,  so 
that  the  violet-blue  and  greenish-blue  are  the  last 
colours  to  disappear,  so  that  one  can  recognise  objects 
of  these  colours  by  their  bright  shimmer  or  sheen, 
whereas  the  red,  orange,  and  other  colours  appear 
dark  purple,  grey,  or  black.  This  change  of  colour  is 
known  as  the  Purkinje  Phenomenon,  from  its  dis- 
coverer. It  shows  us  why  the  sky  still  appears  blue 
in  the  twilight  after  all  other  colours  have  died  away. 
If  one  goes  into  a picture  gallery  at  dusk  all  the  red 
and  orange  colours  appear  nearly  black,  while  the  blue 
colours,  although  “ washed  out,”  appear  whitish  and 
light  in  colour.  If,  on  the  other  hand,  the  light  be 
increased  more  and  more,  the  reds  and  yellows  will 


6o 


PHOTOGRAPHY  IN  COLOURS 


become  gradually  more  visible  until  they  overstep  the 
greens  and  blues,  and  ultimately  become  the  brightest 
of  all  the  colours. 

Mr.  T.  E.  Goodall  has  pointed  out  to  me  that  a 
convolvulus  growing  on  the  western  wall  of  a house  is 
a rich  blue  in  the  morning,  because  it  receives  only 
diffused  light,  but,  as  the  sun  works  round  to  the  west, 
the  colour  ultimately  attains  a magenta-red  just  before 
the  sun  goes  down. 

The  probable  explanation  of  the  Purkinje  phe- 
nomenon lies  in  the  nature  of  the  rods  and  cones  of 
the  retina  itself.  As  we  have  stated  in  § 19,  the  sensitive 
layer  of  the  retina  consists  of  an  immense  number  of 
rods  and  cones  packed  together,  so  as  to  form  a kind 
of  mosaic  when  seen  in  cross-sections  under  the 
microscope.  At  the  foveal  pit  ( i.e . the  centre  of  the 
macula)  there  are  only  cones,  but  the  further  one 
recedes  from  this  spot  the  fewer  are  the  cones,  and  the 
greater  the  number  of  rods.  Now  there  are  several 
ways  of  showing  that  the  cones  are  most  sensitive  to 
the  green,  yellow,  and  orange  rays  (i.e.  between  526/a/a 
and  566/a/a),  whereas  the  rods  are  most  sensitive  to  the 
blue-green  rays  (round  about  570/a/a).  When  one  fixes 
one’s  eyes  on  an  object  as  in  direct  vision,  it  is  the 
cones  of  the  fovea  which  receive  the  sharp  impression. 
This  constitutes  central  vision,  or  cone  vision.  On  the 
other  hand,  all  surrounding  objects  not  fixed  with  the 
eye,  and  which  in  consequence  are  only  imperfectly 
seen,  are  observed  by  indirect  vision,  or  rod  vision , since 
it  is  the  rods  that  greatly  predominate.  The  following 
experiment  will  go  far  to  prove  the  above  statements. 

Eirst  of  all  one  must  have  a projection  lantern  fitted 


THE  SENSITIVENESS  OF  THE  PLATE  6 1 


with  an  arc  light  L,  and  a condenser  C (Fig.  12).  In 
front  of  this  is  a dispersing  (negative)  lens  which 
renders  the  convergent  rays  parallel.  The  parallel  beam 
then  passes  through  the  two  Nicols  Nx  and  N2.  Then 
the  light  passes  through  a round  opening  in  the 


m 


Fig.  12. 


diaphragm  S,  and  finally  through  the  condensing 
lens  L,  and  the  direct  spectroscope  P.  In  this  way  a 
pure  spectrum  can  be  projected  on  to  the  screen  Q. 
The  upper  half  of  the  screen  is  covered  with  two  sheets 
of  red-coloured  tissue  paper,  and  the  lower  half  with 
two  sheets  of  blue-green  paper  of  such  tints  that  the 
red  when  illuminated  is  perceptibly  brighter  than  the 
blue-green.  If  now  one  of  the  Nicols  is  rotated  so  that 
the  light  is  gradually  reduced,  the  blue-green  field  will 
become  brighter  than  the  red  one,  until  at  length  the 
red  field  becomes  invisible ; while  the  blue-green  field 
degenerates  into  a colourless  whitish  sheen.  On 
covering  the  big  sheet  with  black  velvet  and  cutting  a 
small  square  hole  out  of  it  in  the 
centre  (see  Pig.  13)  a small  piece 
of  the  blue-green  and  red  is  to  be 
seen  through  it.  If  now,  while  the 
Nicols  are  arranged  so  that  the 
red  is  just  a little  brighter  than 
the  blue-green,  and  while  one  is 
gazing  at  the  hole,  a second  person  suddenly  lifts  off 


Fed 

-e— 

-Bluish  Greert 


Fig.  13. 


62 


PHOTOGRAPHY  IN  COLOURS 


the  black  velvet  sheet,  the  red  sheet  will  appear  quite 
black,  while  the  white  of  the  upper  (blue-green)  half 
of  the  sheet  will  appear  quite  colourless,  but  wonder- 
fully bright  and  glistening  like  silver. 

This  effect  applies  to  the  photographic  plate  as  well, 
for  it  will  be  found  extremely  difficult  if  not  impossible 
to  photograph  reds,  yellows,  and  yellow-greens  in  their 
natural  colours  in  a dull  light.  This  explains  why 
such  a disproportionately  short  exposure  is  necessary 
when  photographing  with  colour  plates  in  bright  sun- 
shine, and  such  a surprisingly  long  exposure  (compared 
with  the  proportionate  exposure  for  ordinary  black  and 
white  pictures),  when  photographing  with  colour  plates 
in  a very  dull  light  (see  exposure  table  in  the  Appendix 
in  proof  of  this). 


CHAPTER  Y 

METHODS  OF  OBTAINING  PHOTOGRAPHS  IN  COLOUR 

§ 26.  Lippmann’s  Interference  Method. — Among  the 
various  methods  that  have  been  tried  to  produce 
photographs  having  the  natural  colours  of  the  original, 
none  has  excited  more  scientific  interest  than  that  of 
Prof.  Lippmann,  first  shown  in  1890,  for  no  pigments 
of  any  kind  are  employed,  but  the  colours  are  produced 
by  the  interference  of  light  reflected  by  thin  films.  A 
very  fine-grained,  translucent  plate  is  employed,  which 
is  placed,  glass  side  towards  the  lens,  in  a special  dark 
slide.  This  is  filled  with  mercury,  so  as  to  form  a 
mirror  in  contact  with  the  film. 

Let  us  suppose  (Fig.  14)  a series  of  parallel  (plane) 
waves  to  be  refracted  through  a lens  on  to  such  a plate 
in  the  camera.  The  waves  will  pass  through  the  film 
and  be  reflected  at  the  surface  of  the  mirror. 

These  waves  will,  on  their  return,  engender  a number 
of  stationary  waves,  owing  to  their  neutralising  the 
opposing  waves.  Consider  a point  P in  the  film,  which 
is  at  a distance  x from  the  mirror.  Then  the  waves 
about  to  proceed  to  O from  P will  encounter  the  waves 
which  have  been  to  and  are  returning  from  O,  so  that 
at  P we  have  two  sets  of  waves,  the  direct  waves  and 
the  reflected  waves.  They  both  started  in  the  same 


Mercury  tank  forming  mirror. 


64  PHOTOGRAPHY  IN  COLOURS 

phase  at  P,  but  the  reflected  waves  have  travelled  a 
distance  equal  to  2x  further.  Moreover,  on  reaching 
the  mirror,  they  are  found  to  differ  in  phase  by  half  a 
wave-length,  because  the  reflection  took  place  at  the 
surface  of  the  denser  medium,  and,  by  a well-known 


Gelatine  emulsion  film  very  greatly  magnified. 


Fig.  14. 


law,  reflection  at  the  surface  of  a denser  medium  causes 
a retardation  = A/2  in  the  wave. 

Hence  the  path  traversed  by  the  returning  wave 
= 2x  + A/2.  Now  this  must  be  an  odd  number  of 
half-wave  lengths,  provided  that  PO  = ??A/2,  n being 
an  even  number,  since  we  have  the  extra  A/2  to  make 
the  total  an  odd  one.  If  this  is  so,  there  will  be 


Glass. 


Plate  V. 


Section  of  a Lippmann  Photographic  Film,  made  through  the  red  end 
of  the  spectrum  of  arc  light  (x  11,000).  Prepared  and  photographed 
by  Edgar  Senior,  Esq.,  and  reproduced  here  by  his  permission. 
Zeiss’  Apoch.  Obj.  3 mm.  Oc.  2;  yellow  screen.  The  dark  deposits 
of  silver  P,  P1?  P2,  etc.,  occur  at  half  wave-length  intervals  x,  x,  x. 


To  face  p.  65. 


OBTAINING  PHOTOGRAPHS  IN  COLOUR  65 


interference  throughout  the  planes  P and  P1?  because 
Pj  is  separated  by  the  distance  x from  P,  and  so  for 
the  other  planes  P2,  P3,  etc.,  throughout  the  film.  The 
distance  between  these  planes  is  equal  to  half  a given 
wave-length  of  light  for  a definite  line  of  the  spectrum,  so 
that  these  planes  will  vary  for  each  colour,  being  closer 
together  towards  the  violet  end,  and  wider  towards  the 
red.  Wherever  the  reflected  wave  meets  an  incident 
wave  in  the  opposite  phase,  the  trough  of  the  wave 
will  be  filled  up,  so  there  will  be  a calm  region,  and 
it  will  have  no  effect  on  the  silver  bromide  at  that 
spot ; but  where  it  meets  its  opponent  in  the  same 
phase,  the  wave  will  be  intensified  and  have  a strong 
action  on  the  silver  bromide  particles.  If,  therefore, 
the  plate  is  developed  and  fixed,  there  will  be  a series  of 
planes  of  darkened  silver  particles  at  regular  intervals, 
the  width  being  in  strict  proportion  to  the  length  of 
the  wave,  while  in  between  these  will  be  other  planes 
corresponding  to  other  parts  of  the  spectrum.  Dr. 
Neuhaus  first  succeeded  in  stripping  off  a piece  of 
such  a film,  and  making  very  thin  sections,  which, 
when  highly  magnified,  showed  an  appearance  similar 
to  Plate  V. 

The  dark  lines  are  due  to  the  reduced  silver  particles, 
the  bright  lines  to  planes  where  no  action  occurred.  If, 
therefore,  the  film,  when  fixed  and  dried,  be  turned  so 
that  the  eye  sees  the  film  at  or  near  the  angle  of 
reflection,1  the  colours  corresponding  to  those  of  th8 

1 The  angle  of  reflection  is  the  angle  which  the  reflected  ray 
makes  with  a perpendicular  line  drawn  from  the  surface  at  the 
point  of  impact.  It  is  in  the  same  plane  with,  and  equal  to  the 
angle  made  by,  the  incident  ray  with  the  normal  (perpendicular 
line)  above  mentioned. 

F 


66 


PHOTOGRAPHY  IN  COLOURS 


original  object  photographed  will  be  distinctly  perceived. 
By  fixing  a shallow-angled  prism  behind  the  plate  surface 
reflections  will  be  got  rid  of,  and  the  colours  will  be 
brought  out  much  more  vividly.  In  this  way,  not  only 
can  the  solar  spectrum  be  reproduced,  but,  under  favour- 
able circumstances,  landscapes,  flowers,  butterflies,  and 
other  brightly  coloured  objects  may  be  photographed. 
Many  of  the  old  Daguerreotypes  showed  traces  of  the 
natural  colours,  as  can  be  seen  in  specimens  at  the 
present  day  if  observed  at  the  proper  angle,  since 
the  polished  silver  backing  takes  the  place  of  the 
mercury  trough,  but  it  remained  for  Zenker  and  Lipp- 
mann  to  give  the  correct  explanation  of  the  phenomenon. 

Colours  produced  by  interference  were  observed  as 
far  back  as  Sir  Isaac  Newton,  who  described  them  as 
occurring  when  two  glass  plates  are  separated  by  a 
very  thin  film  of  liquid  or  air.  Thus,  if  a slightly 
convex  surface  of  glass  be  placed  on  a perfectly  flat 
surface,  the  thin  film  around  the  point  of  contact  will 
give  rise  to  coloured  circles,  which  are  known  as 
Newton’s  rings.  By  measuring  the  diameters  of  the 
rings,  the  curvature  of  the  glass  may  be  calculated,  or, 
knowing  the  curvature,  the  thickness  of  the  film  can 
be  found,  and  if  monochromatic  light  be  used,  its  wave- 
length may  be  calculated  also.  The  colours  in  the 
plumage  of  many  butterflies  and  birds  and  the  bodies 
of  beetles,  as  well  as  the  exquisite  tints  of  mother-of- 
pearl  and  the  soap-bubble,  are  due  to  interference 
phenomena  and  not  to  actual  pigments.  This  may  be 
seen  by  regarding  these  structures  at  different  angles, 
when  the  colours  will  be  seen  to  vary.  A still  more 
familiar  example  may  be  observed  when  tar  or  petrol 


OBTAINING  PHOTOGRAPHS  IN  COLOUR  67 


is  spilled  and  spreads  over  the  road,  especially  if  it  is 
wet,  so  that  the  liquid  expands  in  a thin  film  over 
the  water. 

§ 27.  Theory  of  Colour  Formation, — If  we  pro- 
ject transparencies  from  a set  of  three-colour  negatives 
on  to  a screen  by  means  of  three  lanterns,  we  must 
place  in  front  of  each  lantern  slide  a coloured  glass 
similar  to  that  used  for  the  corresponding  negative,  i.e. 
we  must  illuminate  the  transparency  from  the  red  filter 
negative  with  red  light,  the  green  with  green  light,  and 
the  blue  with  blue  light,  but  when  we  superpose  the 
transparent  prints,  made  from  each  of  the  negatives, 
we  must  first  colour  each  of  these  prints  not  in  the 
colours  used  for  their  respective  filters,  but  in  the 
complementary  colours  to  these,  i.e . in  colours  which 
transmit  the  other  two  colours  which,  added  to  the 
filter,  made  up  white  light.  Thus  the  negative  taken 
through  the  red  filter  is  printed  in  a colour  transmitting 
green  and  blue,  these  being  the  other  two  colours 
which,  with  red,  form  white  light.  This  colour  is 
cyan-blue,  the  complementary  colour  to  red.  It  is, 
moreover,  a light  greenish-blue,  quite  different  from 
spectrum  deep  blue.  The  green  filter  ’ negative  is 
printed  in  the  complementary  colour  to  green,  viz.  a 
magenta-pink,  and  the  blue  filter  negative  is  printed  in 
the  complementary  colour  to  blue,  viz.  canary-coloured 
yellow. 

The  reason  is  as  follows  : — In  the  former  case  discs 
of  red,  green,  and  blue  lights  are  overlapped,  so  that 
coloured  lights  are  added  to  coloured  lights  (additive 
method),  but  in  superposing  one  print  over  another  we 
are  adding  not  lights  to  lights,  but  opacities  to  opacities, 


68 


PHOTOGRAPHY  IN  COLOURS 


since  each  additional  print  abstracts  part  of  the  light 
transmitted  by  the  first  one.  Thus,  if  one  paints  a 
patch  of  red  pigment  on  a piece  of  white  paper  the  latter 
reflects  light  of  all  colours  and  consequently  appears 
white,  but  the  patch  of  red  pigment  absorbs  the  green 
and  blue  and  only  reflects  the  red  light,  and  therefore  the 
patch  appears  darker  than  the  white.  If  we  now  paint  a 
patch  of  green  and  another  of  blue  over  the  red  patch, 
the  latter  will  appear  black  and  not  white,  whereas  if 
light  is  transmitted  through  transparent  discs  of  three 
primary  colours  in  their  correct  proportions  and  super- 
posed on  a screen,  the  result  will  be  a white  disc. 
Fig.  15  represents  the  additive  effect  of  overlapping 
the  coloured  discs  of  red,  green,  and  blue  lights,  the 
result  being  a white,  or  nearly  white  patch  on  the 
screen,  whereas  Fig.  16  represents  the  subtractive 


Fig.  15. — Diagram  showing 
the  effects  of  additive 
lights. 


Fig.  16. — Diagram  showing 
the  effects  of  subtractive 
colours. 


effect  of  superimposing  discs  of  gelatine  films  or  glass 
stained  with  the  complementary  colours  to  red,  green, 
and  blue,  the  result  being  a black  patch.  The  coloured 
illustration  (Plate  VI.)  shows  the  effect  of  the  super- 
position of  the  colours,  to  which  the  above  figures 
furnish  the  key. 


PLATE  VI. 


Primary  colours.  Diagram,  showing  the  effect  of  superposition  of  Primary  colours.  Diagram  showing  the  effect  of  superposition 

Additive  Lights , as  explained  on  p.  68.  of  “ Subtractive”  Colours , as  explained  on  p.  68. 

To  face  /.  68. 


! 


OBTAINING  PHOTOGRAPHS  IN  COLOUR  69 


In  order  to  make  a three-colour  transparency,  we 
must  therefore  proceed  as  follows  : — A red  filter  negative 
is  taken  and  a contact  transparency  made  by  exposing 
a plate  behind  the  negative,  and  developed  in  the  usual 
way;  the  grey-black  image  of  reduced  silver  is  now 
replaced  by  ferrocyanide  of  iron,  the  metallic  silver 
deposit  acting  as  a mordant,  the  result  being  a greenish- 
blue  colour,  which  fortunately  happens  to  be  the  exact 
complementary  colour  of  the  red  filter.  Two  thin 
transparent  celluloid  films  are  now  coated  with  a 
soluble  gelatine  film  containing  a trace  of  bromide  of 
silver  and  sensitized  like  carbon  tissue  with  bichro- 
mate of  potash.  This  renders  the  parts  affected  by  the 
light  insoluble  in  water.  One  of  these  is  now  placed 
celluloid  side  down  in  contact  with  the  green  filter 
negative,  and  after  the  details  of  the  image  are  quite 
visible  the  exposure  is  stopped.  The  film  is  then 
washed  in  warm  water  to  remove  all  the  unaffected 
gelatine,  fixed  in  “hypo,”  washed  and  dried.  It  is 
then  dipped  in  a crimson -pink  dye  bath,  so  as  to  get 
a pink  print,  the  complement  of  the  green  filter. 
Lastly,  we  obtain  a print  of  the  blue  filter  negative 
with  the  remaining  film  celluloid  side  down,  and,  after 
treatment  in  the  same  way,  it  is  stained  a bright  yellow, 
which  is  the  complementary  colour  of  the  blue ; when 
these  two  prints  are  dry  they  are  mounted  together  in 
correct  register.  The  pink  print  is  cemented  on  to 
the  greenish-blue  transparency,  and  the  yellow  print 
on  the  top  of  all,  the  films  being  placed  face  dowmvards. 
This  is  necessary,  since  the  greenish-blue  print  on  the 
glass  is  a direct  print  made  by  placing  the  sensitive 
side  in  contact  with  the  film  side  of  the  negative, 


70 


PHOTOGRAPHY  IN  COLOURS 


whereas  the  two  celluloid  films  are  printed  through 
the  back  by  placing  the  celluloid  side  next  to  the  film 
of  the  negative,  and  both  are  turned  round  on  finally 
placing  them  together.  This  not  only  secures  the 
prints  being  all  turned  the  right  way,  but  the  two  most 
important  components,  viz.  the  greenish-blue  and  pink 
are  mounted  in  actual  contact,  while  the  third  (yellow) 
print  is  only  separated  by  the  thickness  of  one  film  of 
celluloid,  which  does  not  affect  the  results.  The  three 
pictures  are  then  mounted  behind  glass  and  used  as  a 
lantern  slide,  or  framed  and  hung  in  a window.  If 
the  proper  values  have  been  given  to  the  colours,  the 
final  result,  whether  seen  on  the  screen  or  examined 
by  reflected  light,  is  strikingly  effective  and  realistic. 

The  difficulties  attendant  on  three-colour  photo- 
graphy, and  especially  on  making  all  three  exposures 
at  one  time  and  of  equal  gradation  has  led  to  attempts 
being  made  with  two  colours.  Gurtner  has  invented 
and  patented  a very  simple  process,  which,  while 
ignoring  the  red  element,  still  enables  one  to  produce 
pictures  of  natural  scenery.1 

1 See  § 83,  pp.  152-153. 


CHAPTER  VI 


SINGLE-PLATE  COLOUR  PROCESSES 

§ 28.  Joly’s  Ruled  = Line  Screen  Process. — This  is 
essentially  a three-colour  process,  invented  by  Professor 
Joly  of  Dublin,  in  1897,  but  which  for  various  reasons 
has  not  been  taken  up  commercially.  The  method  is 
as  follows  : — 

A glass  plate  is  ruled  with  a series  of  orange,  blue- 
green,  and  blue  lines,  about  230  in-  aPart>  an(I  repeated 
in  the  above  order  across  the  plate.  This  triple- 
coloured glass  is  placed  just  in  front  of  a sensitized 
plate,  and  a photograph  of  a coloured  object  is  taken 
in  the  camera  and  developed.  The  negative  may 
therefore  be  considered  as  composed  of  three  parts, 
each  corresponding  to  its  particular  line.  A trans- 
parency is  now  made  by  contact,  and  another  plate, 
ruled  with  the  same  number  of  lines,  is  placed  in 
contact  with  it,  only,  instead  of  the  coloured  lines 
being  orange,  blue-green  and  blue,  they  are  now  ruled 
red,  green,  and  blue-violet,  thus  corresponding  to  the 
three-colour  sensations.  The  red  lines  are  adjusted 
to  fall  on  the  image  formed  behind  the  orange  lines, 
the  green  on  the  blue-green,  and  the  blue-violet  on  the 
image  formed  behind  the  blue  image.  It  is  of  prime 


72 


PHOTOGRAPHY  IN  COLOURS 


importance  that  the  lines  are  in  exact  register,  other- 
wise the  whole  aspect  of  the  picture  will  be  changed. 
Therefore  the  lines  on  the  negative  which  were  behind 
the  orange  lines  of  the  screen,  must,  when  viewed 
through  the  positive  transparency,  be  exactly  in  register 
with  the  red  lines  of  the  second  screen,  and  so  for  the 
other  two  colours. 

The  positive  and  second  screen  can  be  placed  in 
register,  and  thrown  on  to  a sheet  by  an  optical  lantern, 
and  a facsimile  in  colours  of  the  original  object  may  be 
seen  by  an  audience  on  the  sheet.  It  is  necessary  that 
the  sheet  should  be  at  some  distance  from  the  audience, 
otherwise  the  lines,  being  highly  magnified,  would  be 
seen.  At  a little  distance  away  the  lines  blend,  and  a 
remarkably  faithful  and  brilliant  image  is  seen.  If  such 
a slide  be  placed  in  front  of  a window  the  colours  can 
still  be  seen,  but  they  vary  according  to  whether  the 
slide  is  looked  at  in  front  or  from  either  side.  Thus  the 
colours  of  a dress  may  appear  of  a rose  colour  when 
observed  obliquely  from  the  right-hand  side,  but  a 
greenish-blue  when  seen  from  the  left  side  of  the 
picture.  This  is  due  to  the  fact  that  the  positive  and 
second  screen  have  their  corresponding  lines  in  register 
when  seen  from  the  front,  but  when  looked  at  obliquely, 
parallax  is  set  up,  so  that  on  the  one  side  the  blue- 
green  lines  predominate,  while  on  the  other  side  the 
red  are  most  seen.  Since  the  red  and  green  lines 
together  produce  yellow  or  orange,  the  green  and  blue- 
violet,  blue,  and  the  blue-violet  and  red,  crimson,  it  will 
be  seen  that  all  shades  of  colour  can  be  reproduced, 
although  the  mixing  of  lights  and  the  mixing  of  pigments 
will  not  produce  the  same  results. 


SINGLE-PLATE  COLOUR  PROCESSES  73 


§ 29.  Comparison  between  the  Various  Screen 
Plates.- — We  may  divide  all  the  single-plate  colour 
processes  into  three  classes  according  to  the  nature  of  the 
colour-screen.  It  is  unnecessary  to  describe  the  methods 
of  manufacture  as  they  are  extremely  complicated,  and 
what  the  reader  really  wants  to  know  is  how  the  various 
screens  affect  the  appearance  of  the  picture.  All  the 
screens,  however,  have  one  feature  in  common : they 
all  consist  of  a pattern  composed  of  the  three  primary 
colours,  red,  green,  and  blue-violet,  which  transmit 
their  respective  coloured  lights.  By  drawing  up  a table 
we  can  best  review  all  the  screen  plates  that  have  so 
far  appeared,  omitting  only  those  that  have  been 
abandoned. 

It  will  be  seen  from  the  accompanying  table  that  the 
plates  in  Class  I.  are  distinguished  from  those  in  Classes 
II.  and  III.  by  being  “ regular  ” in  texture,  whereas  the 
latter  are  “ irregular.” 

A “ regular  ” plate  is  one  which  is  ruled  or  impressed 
with  a regular  series  of  discs,  lines,  or  squares,  of  which 
the  Paget  and  Omnicolore  are  good  examples. 

An  “ irregular  ” plate  is  one  in  which  the  coloured 
particles  are  strewn  haphazard  over  the  plate.  The 
Autochrome  is  the  best  known  example  of  this  class. 

The  “ regular  ” plates  all  possess  extreme  softness 
and  brilliancy  of  colouring.  The  colours  are  very  rich 
and  vivid,  and  produce  a woven  silk-like  appearance 
due  to  the  symmetry  of  the  pattern.  If  a suitable  sub- 
ject be  selected  and  the  plate  correctly  exposed  and 
developed,  the  result  is  remarkably  fine,  and  cannot  be 
excelled  (if  equalled)  by  any  other  kind  of  plate.  On 
the  other  hand,  the  range  of  colours,  and  especially  the 


Characteristic  Features  of  the  Principal  Single-Plate  Colour  Screens. 


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SINGLE-PLATE  COLOUR  PROCESSES  75 


tones  and  shades,  are  more  restricted,  and  the  colours 
less  true  to  nature  than  in  the  case  of  an  Autochrome 
similarly  treated.  This  is  partly  due  to  the  fact  that 
all  “ regular  ” plates  partake  more  or  less  of  the  nature 
of  a diffraction  grating,  and  are  liable  to  give  rise  to 
imperfectly  formed  and  impure  spectra  which  interfere 
seriously  with  the  general  appearance  of  the  picture. 
The  conspicuousness  of  these  spectra  is  in  direct  pro- 
portion to  the  fineness  of  the  rulings.  (As  discs  do  not, 
like  rulings,  give  rise  to  spectra,  the  Thames  plate  is 
free  from  this  defect.)  Another  drawback  is  that  the 
colours  tend  to  change  into  their  complementaries  or 
into  mixtures  of  one  or  more  of  the  adjacent  colours 
when  the  picture  is  viewed  sideways.  This  is  owing  to 
parallax,  and  is  specially  noticeable  in  those  plates  in 
which  the  screen  is  separated  from  the  sensitive  film. 

§ 30.  Parallax — may  be  defined  as  the  displacement 
of  one  object  with  respect  to  another  when  viewed  from 
different  positions.  If,  for  example,  you  place  a red,  a 
green  and  a blue  marble  to  represent  the  three  coloured 
discs  side  by  side  on  the  table  at  right  angles  to  the 
line  of  view,  and  put  a white  marble  a couple  of  inches 
in  front  of  the  centre  (green)  one  and  then  look  along 
the  table  at  it  a couple  of  feet  away,  the  white  marble 
will  hide  the  green  one.  If  you  move  a little  to  the 
right  it  will  hide  the  red  one.  If  you  move  to  the  left 
it  will  hide  the  blue  marble.  This  is  due  to  parallax, 
and  the  amount  of  parallax  increases  directly  with  the 
distance  between  the  two  planes.  It  is  this  parallactic 
displacement  which  enables  astronomers  to  measure 
approximately  the  distance  of  some  of  the  stars,  when 
the  earth  is  first  at  one  end  and  six  months  afterwards 


76 


PHOTOGRAPHY  IN  COLOURS 


at  the  other  end  of  its  elliptical  orbit  as  it  travels 
round  the  sun. 

The  Autochrome  plate  gives  rise  to  a purer  and  wider 
range  of  colours.  As  the  film  is  very  thin  there  is  no 
parallax,  and, like  all  other  “irregular  plates,” cannot  give 
rise  to  diffraction  spectra  even  when  the  transparency 
is  illuminated  by  the  arc  or  clear  bulb  electric  light. 
The  grain  is  so  fine  that  complex  colours  such  as  white, 
greys,  gold,  silver,  flesh,  ivory,  shagreen,  etc.,  can  be  very 
faithfully  reproduced,  and  these  most  of  the  “ regular  ” 
plates  fail  in  rendering  properly. 

§ 31.  Jougla’s  “ Omnicolore  ” Plate. — The  colour- 
screen  consists  of  a series  of  parallel  blue  rulings  or 
stripes,  the  intervening  spaces  being  filled  by  green  colour 
surrounding  red  squares  (Plate  VII.).  The  breadth  of 
the  blue  stripes  = 0*04  mm.  The  breadth  of  the  green 
spaces  007  mm.,  and  that  of  each  red  square  0-05  mm. 
Hence,  if  we  divide  up  the  blue  stripes  into  as  many 
areas  as  there  are  red  squares  or  green  intervals,  we 
shall  find  the  ratio  of  the  red,  green,  and  blue  areas  to 
be  as  3 : 4 : 5,  the  blue  areas  being  nearly  twice  the  size 
of  the  red  ones,  and  the  green  intermediate  between  the 
two.  The  relative  sizes  correspond  to  the  apparent 
brightness  of  each  of  these  colours  to  the  eye.  This 
difference  is  only  apparent,  as  it  is  corrected  by  the 
filter,  which  is  of  a light  canary  yellow  colour,  and  is 
slightly  more  rapid  than  the  Autochrome  one.  The 
film  is  tough. 

§ 32.  Dufay’s  “ Dioptichrome  ” Plate  closely 
resembles  the  Omnicolore  both  in  appearance,  method 
of  developing,  and  in  results.  The  “ First  black  con- 
dition ” (see  § 25,  p.  59)  is  very  perfectly  fulfilled.  The 


3?late  VII. 


Thames  ” Screen  x 100. 


Omnicolore  ” Screen  x 100. 


To  face  p.  76. 


SINGLE-PLATE  COLOUR  PROCESSES  77 


colours  are  therefore  remarkably  true  to  nature.  The 
colour-screen  consists  of  a series  of  parallel  green 
rulings,  the  intervening  spaces  being  filled  by  alternating 
red  and  blue  squares  (Plate  VIII.).  The  breadth  of 
the  green  stripes  = 0*06  mm.  That  of  the  blue  squares 
= 0-06  mm.,  and  the  red  0'07  mm.  Like  the  Omni- 
colore and  Thames,  the  colours,  if  strong  and  vivid,  have 
a charmingly  artistic  texture  as  if  painted  on  woven 
silk.  The  plates  are  said  to  be  more  rapid 1 than  the 
Autochrome  in  the  proportion  of  5"  to  6|  (see  § 39),  and 
the  film  will  bear  rougher  handling  than  the  Autochrome, 
but  less  than  the  Omnicolore  or  Thames  plates.  The 
colour  filters  of  the  Omnicolore  and  Dufay  are  practi- 
cally the  same,  and  therefore  interchangeable,  but  they 
will  not  be  correct  for  either  the  Autochrome  or  Thames 
plates,  the  former  having  a pinker  tint  and  the  latter 
a pale  yellow  one.  The  positive,  owing  to  its  great 
transparency,  is  admirably  suited  to  lantern  projection. 

§ 33.  Thames  Screen  Plate. — This  plate,  invented 
and  made  in  London  by  the  Thames  Colour  Plate 
Company,  has  a screen  impressed  with  rows  of  alter- 
nating red  and  green  circular  discs.  The  interstices 
are  filled  in  with  a violet-blue  dye,  so  that  the  entire 
plate  is  covered  with  the  three  colours,  and  presents  no 
blank  spaces.  See  Plate  VII. 

The  colour-filter  is  a pale  canary  yellow.  Since  the 
colours  are  much  more  transparent  than  the  starch 
cells  of  the  Autochrome  plate,  the  rapidity  is  corre- 
spondingly increased.  Also  a lighter  colour-filter  being 
used,  shutter  exposures  as  short  as  N sec.  can  be  made 

1 According  to  the  author’s  experience  they  are  somewhat  less 
rapid  (see  Appendix  table). 


78  PHOTOGRAPHY  IN  COLOURS 

under  favourable  conditions  with  a stop  of  F/5*6, 
whereby  objects  in  motion  can  be  photographed. 

§ 34.  Combined  and  Separate  Screen  Plates 
compared. — The  Thames  Colour  Plate  Co.1  issue 
plates  of  both  kinds,  and  each  has  its  advocates. 
The  combined  plate  in  which  the  sensitive  film  is 
permanently  attached  to  the  screen  is  easier  to  work, 
as  the  positive  is  obtained  direct  by  immersion  into  a 
dissolving  agent  such  as  acid  bichromate.  Trouble 
from  parallax,  etc.,  as  described  above,  does  not  arise 
in  the  combined  plate. 

The  separate  method  (i.e.  one  in  which  the  colour- 
screen  is  on  a separate  glass  from  the  panchromatic  film) 
is  perhaps  a little  more  complicated  in  the  working, 
since  after  taking  the  negative  a positive  must  be 
made  from  it,  and  carefully  adjusted  to  a screen. 
The  registration  of  the  latter  requires  a consider- 
able amount  of  care  to  perform  correctly.  Owing  to 
the  slight  separation  of  the  screen  and  film,  parallax 
readily  appears ; nevertheless  the  separate  method 
has  many  points  in  its  favour.  If  the  negative  is 
defective  we  may  substitute  another  panchromatic 
plate  and  make  a second  exposure  at  a very  much 
smaller  cost.  Also  we  may  use  the  negative  to  print 
off  on  P.O.P.  any  number  of  copies,  and  further,  as  the 
screen  causes  the  entire  picture  to  be  broken  up  by  its 
almost  invisible  pattern,  the  prints  possess  a peculiar 
charm  of  their  own.  Enlargements  in  monochrome 
may  be  made  from  them,  or  they  can  be  reproduced  in 

1 This  company  no  longer  issues  plates.  But  an  improved 
plate  is  now  sold  by  the  Paget  Prize  Plate  Co.,  Ltd.  (see  Paget 
plate). 


Plate  VIII. 


Dioptichrome  ” Screen  x 100. 


To  fact  p.  78. 


SINGLE-PLATE  COLOUR  PROCESSES  79 


colour,  as  will  be  described  later.  It  is  obvious  that 
“ irregular”  plates  can  never  be  worked  by  the  separate 
method,  as  it  is  impossible  to  readjust  the  positive  and 
screen  into  register  again.  In  the  separate  method 
one  can  always  use  the  negative  for  printing  pur- 
poses in  the  same  way  as  any  other  negative,  and 
then  make  a transparency  on  a slow  process  or  lantern 
plate  for  the  purpose  of  binding  up  with  a screen  for  a 
colour  transparency.  As  the  screens  are  exactly  alike 
any  one  of  them  may  be  used.  The  uncombined  or 
“ separate  plates  ” are  twice  as  rapid  as  the  combined 
ones.  In  both  cases  the  films  will  stand  fairly  rough 
usage  compared  with  the  Autochrome. 

Quite  recently  a method  of  getting  rid  of  all  parallax 
has  been  devised  by  the  makers  of  the  Thames  plate. 
Instead  of  making  a positive  transparency  and  binding 
it  up  in  register  with  a colour-screen,  they  coat  the 
colour-screen  with  a transparent  (and  consequently 
extremely  thin)  and  very  slow  emulsion — in  other 
words,  a lantern-slide  emulsion.  One  of  these  is  placed 
film  to  film  with  the  original  negative  and  carefully 
registered  by  candle-light  in  the  dark  room,  a ground 
glass  being  placed  in  front  of  the  candle  to  distribute 
the  light.  The  registration  must  toe  made  in  the  comple- 
mentary colours  of  the  subject , e.g.  yellow  flowers  must  be 
registered  as  blue,  and  green  leaves  as  red,  etc.  An 
exposure  is  then  made  to  strong  light  and  the  colour- 
screen  plate  developed  and  fixed  in  the  ordinary 
manner.  In  this  way  picture  and  screen  are  combined 
in  a single  transparency.  These  coated  colour-screens 
are  about  to  be  placed  on  the  market  by  the  Company. 

§ 35.  Paget  Plate. — This  make  of  colour-plate  is 


8o 


PHOTOGRAPHY  IN  COLOURS 


produced  exactly  on  the  same  lines  as  the  Thames  plate 
already  described,  in  fact  it  is  merely  an  improvement 
on  the  latter,  and  like  it  is  issued  in  two  forms,  the 
separate  and  the  combined  plate. 

In  the  former  case  the  panchromatic  sensitive  plate 
and  the  colour-screen  are  issued  separately  and  are 
placed  with  their  films  in  contact  in  the  dark  slide  as 
tightly  as  possible,  so  that  the  two  films  have  nowhere 
any  air  space  between  them,  otherwise  parallax  will  be 
set  up  and  the  result  will  not  be  satisfactory.  For  this 
purpose  a hinged  back  is  the  best,  and  care  should  be 
taken  that  the  spring  which  holds  the  plate  down  is  a 
strong  one.  The  glass  side  of  the  colour-screen  must, 
of  course,  be  placed  facing  the  lens.  After  exposure, 
the  plate  is  developed  in  the  ordinary  way,  fixed,  and 
dried,  and  a positive  made  from  it  in  a printing-frame, 
having  the  glass  side  of  the  screen  facing  the  light 
exactly  as  in  the  former  case.  After  development, 
fixing,  and  drying,  the  positive  is  adjusted  carefully 
behind  a viewing  colour-screen  and  bound  up  like  a 
lantern  slide. 

The  other  form  of  plate  has  the  screen  coated  with  a 
panchromatic  emulsion  in  the  same  way  as  an  Auto- 
chrome or  Dufay  plate. 

The  chief  differences  between  the  Paget  and  the 
Thames  plates  are;  (1st),  the  screen  is  formed  of  small 
coloured  squares  instead  of  circles  which  are  slightly 
smaller  than  the  latter,  being  each  about  3^  inch  in 
diameter  instead  of  ^ inch.  (2nd)  Two  screens  are 
employed,  one  for  taking  the  negative  (taking- screen), 
the  other  for  binding  up  with  the  positive  copy  (viewing 
screen).  These  two  screens  are  identical,  save  that  the 


SINGLE-PLATE  COLOUR  PROCESSES  8 1 


colours  of  the  squares  are  not  quite  the  same.  The 
taking-screen  is  of  a pale  indigo  colour,  with  a faint 
trace  of  green,  the  other  also  a pale  indigo,  but  in- 
clining very  slightly  towards  a brown  shade.  But 
they  are  very  nearly  alike. 

The  filter  used  is  cut  from  a thin  sheet  of  gelatine 
stained  with  Aurantia,  or  some  such  yellow  dye.  In 
cutting  out  a disc  to  place  between  the  lenses  of  the 
combination  it  is  advisable  to  cut  it  out  with  a pair  of 
scissors,  holding  the  gelatine  between  two  pieces  of 
paper  in  order  to  prevent  the  fingers  from  marking  it, 
as  the  gelatine  is  very  easily  smeared  by  the  fingers, 
and  the  marks  cannot  be  removed  afterwards.  If  the 
inner  flange  of  the  lens  be  pressed  on  a piece  of  paper, 
it  is  quite  easy  to  cut  round  inside  the  ring  so  formed 
with  a pair  of  scissors. 

The  exposure  (see  Tables)  is  about  |th  or  Jth  that 
of  a Lumiere  Autochrome,  and  corresponds  to  15 
Watkins  speed  number;  or  F24  Wynne,  with  plate- 
screen  in  position.  For  an  open  landscape  in  good 
light  at  F/8,  \ second  exposure  is  enough.  For  an 
open-air  portrait  (entire  figure)  in  sunlight,  1 second 
with  F/8,  or  3 seconds  with  head  and  shoulders  only, 
in  diffused  light. 

As  there  is  a dip  in  the  spectrum-curve  of  the  plate, 
viz.  in  the  region  of  the  green,  a developing-lamp  may 
be  used  if  the  light  be  screened  by  three  sheets  of 
yellow  and  three  of  green  Virida  paper  as  for  other 
single  colour-plates,  but  on  no  account  must  a red 
light  be  used. 

§ 36.  Development. — Any  developer  which  gives 
rich  black  tones  may  be  employed,  but  the  makers 

G 


82 


PHOTOGRAPHY  IN  COLOURS 


recommend  1 : 30  Rodinal.  Development  should  be 
complete  in  two  minutes.  It  is  advisable  to  cover  the 
dish  over  with  a card  during  development,  only  exposing 
the  plate  to  the  light  for  a second  (or  less)  after  the 
expiration  of  15  seconds  from  pouring  on  the  developer, 
so  as  to  know  the  instant  the  picture  begins  to  appear. 
Once  this  is  recognised,  the  time  can  be  multiplied 
by  the  factor-number,  and  the  development  completed 
in  darkness.  The  great  point  is  to  obtain  a clean, 
brilliant  (plucky)  negative  entirely  free  from  fog, 
and1  with  clear  shadows.  Otherwise  a dull  positive 
will  result,  and  weak  or  disappointing  colour- effects 
when  the  positive  is  bound  up  with  the  viewing- 
screen. 

When  the  negative  is  dried,  a transparency  copy  is 
made  by  contact  in  a printing-frame  with  any  Ordinary 
plate.  The  Paget  Company  issue  a special  fine-grain 
slow  plate  for  the  purpose,  which  has  a very  thin  film 
similar  to  that  of  a lantern  plate.  This  is  developed 
in  the  ordinary  way,  using  a red  light.  A developer 
which  gives  a dense  black  image  is  the  best.  For  this 
purpose  Metol  or  Metol  - hydroquinone  is  recom- 
mended. The  exposure  averages  about  15  seconds 
with  an  ordinary  candle  at  1 foot,  or  5 seconds  with 
a 16  candlepower  electric  light  at  3 feet. 

It  is  best  to  use  a hinge  double  back  to  hold  the 
two  plates,  since  there  is  more  room  than  in  the  case 
of  a solid  slide;  and,  besides  this,  the  metal  division 
is  furnished  with  a spring  which  presses  the  plates 
together.  If  the  slides  have  a very  shallow  rebate, 
or  if  metal  sheaths  are  used  to  hold  the  plates,  it  is 
necessary  to  procure  extra  thin  taking-screens.  These 


SINGLE-PLATE  COLOUR  PROCESSES  83 

4 

can  now  be  obtained  from  the  Paget  Company  in  the 
place  of  the  thick  ones. 

The  essential  point  is  to  get  absolute  contact  between 
the  two  plates,  and  for  this  reason  a strong  spring  to 
press  them  together  is  a sine  qua  non.  If  they  are 
not  firmly  squeezed  together  you  will  hardly  get  any  colour 
at  all  in  the  finished  picture. 

The  makers  do  not  recommend  either  intensification 
or  reduction  of  the  negative,  so  that  any  modification 
in  the  density  must  be  effected  by  altering  the  exposure 
or  development  of  the  positive. 

For  fixing,  use  hypo  6 oz.,  metabisulphate  of  potash 
\ oz.,  and  water  20  oz. 

When  finally  binding  up  the  positives  a sheet  of 
fine-ground  glass  is  recommended.  Some  trans- 
parencies, however,  are  better  without  it.  When  the 
viewing-screen  is  placed  behind  the  transparency,  a 
coloured  moiiA  pattern  will  be  noticed.  As  the  screen 
is  shifted,  this  pattern  will  be  seen  to  grow  larger  and 
larger,  until  it  finally  disappears.  This  is  the  moment 
when  the  colours  are  correct. 

Notwithstanding  all  that  has  been  written  to  the 
contrary,  there  can  be  no  doubt  that  combined 
plates  are  much  easier  to  work,  and  give  more  satis- 
factory results  than  the  separate  method.  Moreover, 
they  give  richer  colours  with  more  body,  and  run  no 
risk  of  getting  out  of  register,  a fault  which  is  very 
difficult  to  adjust  afterwards. 

On  the  other  hand,  it  cannot  be  denied  that  the 
separate  plates  possess  three  distinct  points  in  their 
favour.  (1)  They  are  three  times  as  rapid  as  a Lumiere 
plate,  which  fact  allows  of  a moving  object  being  taken 


84  PHOTOGRAPHY  IN  COLOURS 

with  full  aperture.  (2)  Being  very  transparent,  they 
are  specially  suitable  for  lantern  projection.  (3)  Any 
number  of  copies  can  be  taken  by  making  fresh 
transparency  copies  and  binding  them  up  with  new 
viewing-screens. 

§ 37.  Combined  Paget  Plate.1 — This  requires  no 
special  description.  It  is  exposed  and  developed  pre- 
cisely in  the  same  way  as  an  Autochrome  or  Dufay 
plate,  the  picture  being  reversed  in  an  acid  bichromate 
solution  afterwards,  and  then  redeveloped  in  full  day- 
light (see  instructions  for  developing  Autocbrome 
plates). 

As  the  grating  is  broken  up  by  the  ruling  which 
forms  microscopic  squares,  the  spectra  are  barely 
perceptible  even  with  artificial  light. 

§ 38.  The  Lumiere  “ Autochrome,,  Plate. — This 
beautiful  process,  patented  by  A.  Lumiere  et  Fils 
of  Lyons  in  1904,  depends  on  a colour-screen  built  up 
of  starch  grains  dyed  with  the  three  primary  colours, 
red,  green,  and  blue-violet,  and  overlaid  with  a thin  pan- 
chromatic sensitized  film.  The  grains  are  of  ordinary 
potato  starch,  varying  between  0*01  and  0*02  mm.  The 
smallest  cells  are  about  the  size  of  a white  blood- 
corpuscle,  so  that  the  grains  are  just  within  the  limit  of 
perception.  There  are  about  four  million  grains  per 
square  inch.  When  an  Autochrome  slide  is  projected 
on  to  a lantern  sheet  the  coloured  grains  are  invisible 
to  the  audience  a few  yards  away  (see  Plates  IX. 
and  X.). 

After  dyeing,  the  starch  cells  are  mixed  together  very 
intimately  (so  as  to  avoid  “ clumping  ”)  in  the  proportion 

1 This  plate  is  not  yet  on  the  market  (November,  1915). 


bJO  ft 


PLATE  IX. 


rain  Filter.  Three-colour  print,  copied  from  a Lumiere  Autochrome 
late,  showing  the  dyed  grains  ; magnified  700  diameters. 


To  face  p.  84. 


SINGLE-PLATE  COLOUR  PROCESSES  85 

of  four  green  to  three  red  and  two  blue.  The  layer  of 
cells  is  flattened  by  rollers  to  fill  up  the  interstices,  and 
then  varnished.  Since  the  three  colours  combine  to 
give  the  effect  of  white  to  the  eye,  an  Autochrome  plate 
should  resemble  an  ordinary  dry  plate.  As  a matter  of 
fact,  the  screen  has  a pale  salmon-pink  colour  when 
held  up  to  the  light. 

§ 39.  Relative  Speeds. — The  following  table  by 
Mees  and  Pledge  gives  the  relative  speeds  of  emulsion 
and  screens,  and  the  exposure  speeds  behind  their 
respective  filters  for  the  respective  plates  : — 


Autochrome. 

Paget. 

Omnicolore. 

Dufay. 

Emulsion  speed  (Watkins) 

35 

120 

22 

13 

Screen  factor  „ 

12 

8 

7 

5 

Filter  factor  ,, 

2 

ij 

H 

1^ 

Effective  speed  „ 

1* 

41 

■*"2 

2 h 

2 

so  that  if  the  Paget  plate  requires  1 sec.  exposure,  the 
Autochrome  requires  about  3 secs.,  the  Omnicolore 
plate  2 secs.,  and  the  Dufay  a little  less. 


CHAPTER  YII 


SINGLE-PLATE  PROCESSES  DIAGRAMMATICALLY 
EXPLAINED 

§ 40.  Von  Miibl’s  Diagram. — As  some  readers  can 
understand  the  subject  better  by  means  of  a diagram 
than  by  words  alone,  Fig.  26,  reproduced  from  a 
diagram  by  Von  Hiibl,  shows  in  diagrammatic  section 
the  action  of  light  passing  through  the  glass  and 
falling  on  the  three-colour  layer  of  the  screen 
plate.  This  diagram  may  be  applied  when  con- 
sidering the  case  of  any  of  the  classes  of  colour- 
screen  plates,  which,  though  differing  in  construction, 
are  all  identical  in  principle.  They  all  agree  in 
having  lines,  patches,  or  spots  of  red,  green,  and  blue 
in  more  or  less  equal  proportions  over  the  entire 
plate  (r.  gr.  bl.,  Fig.  17).  If  such  a plate  is  now 
exposed  in  a camera,  with  the  glass  side  turned  towards 
a coloured  object,  we  shall  obtain  the  following  result : 
A cinnabar  red  object  which  absorbs  green  and  blue  will 
only  emit  red  rays  ; these  will  pass  through  the  red 
elements  of  the  screen,  and  will  act  on  the  particles  of 
silver  bromide  behind  them,  and  the  film  at  that  spot 
will  become  blackened  in  the  course  of  development. 
The  red  rays  will  be  absorbed  by  the  green  and  blue 
elements  of  the  screen,  and  so  no  change  will  be  seen 
in  the  film  behind  them.  If  the  plate  be  now  developed 


Plate  X. 


“ Autochrome  ” Screen  x 100. 


Warner  Powrie  ” Screen  x 100. 


To  face  / . £6. 


Bromide  Emulsion  Layer 


SINGLE-PLATE  PROCESSES  EXPLAINED  87 


S'* 


r 

gf* 

Cinnabars 

bl 

r 

E 

bl 

- 

Yellow-Greens 

r 

<- 

Blues 

S'3 

|bT 

r 

E 

|bT 

« 

Yellows 

| p 

E 

[bl 

<- 

Blue-Greens 

[P 

E 

|bT 

- 

Purples 

Ip 

E 

[bf 

- 

White 

p 

i 

Black 

S'* 

bl 

| p 

E 

[bT 

i 

Greys 

Ip 

E 

r 

- 

Orange  -Yellows 

Ip 

E 

IbT 

— 

L Orange  Yellows 

1 

- 

- 

Browns 

1 

9r 

|bl 

£. 


Fig.  17, 


88 


PHOTOGRAPHY  IN  COLOURS 


and  fixed  we  shall  find  all  the  red  elements  covered 
with  blackened  silver,  but  the  blue  and  green  ones  will  be 
transparent,  and  seen  together  will  give  rise  to  a bluish- 
green  colour  (see  right-hand  diagram).  If  the  object 
is  green,  the  rays  will  be  absorbed  by  the  red  and  blue 
elements,  and  the  film  behind  the  green  elements  will 
become  blackened,  so  that  after  development  and  fixa- 
tion the  plate  will  show  a purple-red  colour.  In  the 
same  way,  a yellow  object  (which  appears  yellow 
because  it  absorbs  all  the  blue  rays)  emits  green  and 
red  rays,  and  these  will  be  absorbed  by  the  blue 
elements  and  will  pass  through  the  red  and  green  ones. 
The  result  will  be  that  the  silver  behind  the  red  and 
green  elements  will  be  blackened  by  the  developer,  and 
the  plate  will  appear  blue  at  that  spot.  If  the  object  is 
a brown  one  (see  bottom  space),  it  will  emit  a large 
number  of  red  rays,  a small  number  of  green  rays,  and 
hardly  any  blue  at  all,  so  that  the  red  elements  will 
allow  about  half  the  light  to  go  through,  the  green 
about  a quarter,  and  the  blue  perhaps  an  eighth. 
After  development  and  fixing,  the  film  behind  the  red 
elements  will  appear  dark  grey,  that  behind  the  green 
elements  a light  grey,  and  that  behind  the  blue  hardly 
changed  at  all,  with  the  result  that  after  development 
and  fixing,  the  negative  will  show  a grey  yellowish-green 
colour.  The  fixed  negative,  therefore,  will  always 
appear  in  the  complementary  colour  to  the  objects 
photographed.  If  after  development  but  without 
fixing  the  negative  be  placed  in  a bath  of  acid 
permanganate  of  potash,  or  acid  bichromate  of  soda, 
the  whole  of  the  blackened  silver  deposit  is  dis- 
solved away.  If  now  the  plate  is  exposed  to  daylight 


SINGLE-PLATE  PROCESSES  EXPLAINED  89 

and  redeveloped,  the  whole  of  the  silver  bromide 
which  has  not  been  acted  on  originally,  will  be  reduced 
to  blackened  silver,  so  that  the  image  will  now  be 
reversed,  and  all  those  parts  which  were  blackened 
during  the  first  development  will  be  now  more  or  less 
transparent.  In  this  way  the  picture  will  appear  in 
the  colours  of  the  objects  photographed.  This  can  be 
readily  seen  by  comparing  the  right  and  left  diagrams 
of  Fig.  17.  A black  object  will  reflect  hardly  any 
light,  so  that  the  developed  negative  will  be  nearly 
transparent  in  the  region  of  the  image  if  the  plate  is 
fixed.  If  it  is  not  fixed  but  exposed  to  light  and 
redeveloped,  then  all  the  bromide  of  silver  will  be 
reduced  to  blackened  silver,  and  the  positive  will  show 
a black  image.  If,  on  the  other  hand,  a white  object 
is  photographed,  the  light  will  pass  through  all  three 
colours,  red,  green,  and  blue,  with  the  result  that  the 
image  will  appear  quite  black  on  development.  In  the 
reversal  bath  this  image  will  be  nearly  completely  dis- 
solved, so  that  on  redeveloping  there  will  be  next  to 
nothing  left  to  develop,  and  the  image  will  appear 
white.  In  fact,  we  may  say  generally  that  after  the 
second  development  there  is  nothing  left  to  fix  in  the 
hypo. 

§ 41.  The  First  Black  Condition. — McDonough 
has  stated  that  the  perfect  screen  plate  is  one  in  which 
the  colours  are  so  perfectly  balanced  that  when  viewed 
as  a transparency  it  is  entirely  free  from  colour.  This 
he  calls  the  first  black  condition.  There  is  no  plate 
which  entirely  comes  up  to  this  standard,  but  the 
Dufay  and  the  Lumiere  line  screens  are  practically 
neutral,  while  the  Paget  is  a pale  greenish-indigo. 


90 


PHOTOGRAPHY  IN  COLOURS 


Some  varieties  of  colour  plates  fall  lamentably  below 
this  standard,  and  cannot  even  reproduce  the  spectrum 
colours  so  that  they  can  be  recognised.  Such  plates 
are  worse  than  useless,  and  tend  to  bring  the  art  into 
discredit. 

§ 42.  The  Second  Black  Condition. — If  the  first 
condition  be  fulfilled,  it  is  further  necessary,  in  order 
to  produce  a perfectly  compensated  plate,  that  it  should 
comply  with  the  second  black  condition,  viz.  that  the 
colour-filter  should  have  such  power  of  absorption  that 
when  a grey  object  is  photographed  an  identical  silver 
deposit  should  be  produced  behind  each  of  the  three 
coloured  elements.  Several  makes  of  plates  are  satis- 
factory in  this  respect. 

§ 43.  How  the  Appearance  of  White  is  produced 
on  an  Autochrome  Colour=  Plate. — It  may  be  ob- 
served by  any  one  that  if  a piece  of  sensitive  film  be 
stripped  off  a colour-plate  leaving  the  coloured  starch- 
grains  on  the  plate,  and  the  latter  be  held  up  to  the 
light  and  examined,  the  stripped  portion  does  not 
appear  white  (as  one  would  expect)  owing  to  the 
combined  sensation  produced  by  the  three  primary 
colours;  but,  instead,  a pale  pink,  grey,  or  greenish 
tint  is  seen.  Why,  then,  does  a snow-mountain,  a 
white  shirt-front,  or  a collar,  come  out  a pure  white 
in  the  transparency?  The  reason  appears  to  be  as 
follows : — If  an  Autochrome  positive  be  examined  under 
a high-power  microscope,  and  if  the  grains  be  carefully 
focussed  up,  they  will  be  seen  as  perfectly  clear  grains 
wherever  a white  object  has  left  its  image  on  the 
positive. 

If,  now,  the  objective  be  very  gently  racked  away 


SINGLE-PLATE  PROCESSES  EXPLAINED  9 I 


from  the  grains  by  means  of  the  fine  adjustment,  an 
immense  number  of  very  fine  black  reduced  silver 
particles  will  be  seen  covering  the  field.  It  is  this  veil 
of  fine  pigment  particles  which  causes  the  sensation  of 
white,  and  the  finer  the  silver  particles,  and  the  more 
evenly  distributed  they  are,  the  whiter  will  the  image 
appear.  This  interesting  discovery  was  first  made  by 
Professor  A.  Forster,  of  Berne,  in  1910.  There  can 
be  no  doubt  that  this  is  the  cause  of  the  impression  of 
white  to  the  eye.  The  professor  ascribes  the  white 
appearance  to  the  black  particles  forming  a grating 
(Raster),  but  after  a careful  examination  of  a great 
number  of  specimens  under  the  microscope,  the  writer 
came  to  the  conclusion  that  a grating  cannot  possibly 
be  formed  by  these  pigment  particles,  and  the  action 
of  the  particles  in  producing  the  sensation  of  white 
light  requires  some  other  explanation.  This  the  writer 
has  attempted  to  do  as  follows : if  we  take  a plate 
from  which  the  gelatine  layer  has  been  stripped  off,  we 
shall  find  that  the  light  passes  through  the  red,  green, 
and  blue  starch  grains  in  the  form  of  train  waves 
having  regular  periods.  When,  however,  the  light  of 
a white  object  has  acted  on  the  silver  emulsion  of 
a plate  exposed  in  the  camera,  it  causes  a fine  pre- 
cipitate of  black  reduced  silver  particles  over  each  of 
the  starch  grains.  In  the  transparency,  these  particles 
break  up  and  scatter  the  light,  so  that  it  no  longer 
reaches  the  eye  as  periodic  waves,  but  in  the  form  of 
scattered  waves  of  red,  green,  and  blue,  which,  mixing 
together,  leave  on  the  eye  the  impression  of  white. 
This,  in  the  opinion  of  the  writer,  is  why  a white  object 
can  be  photographed  as  white  (see  § 6 in  Chapter  I., 


92 


PHOTOGRAPHY  IN  COLOURS 


entitled  “ On  the  Nature  of  White  Light  ”).  For  a com- 
plete account  of  the  microscopic  appearance  see  Prof. 
Forster’s  very  interesting  paper,  entitled  “ Wie  ensteht 
das  Weiss  auf  Dr.  Lumiere’s  Autochromplatten.” 
Zeitschrift  fur  wissenschaftliche  Photographie,  Band 
IX.,  Heft  9,  1911,  Leipsig. 


CHAPTER  VIII 

PRACTICAL  DETAILS  OF  THE  WORKING  OF  SINGLE 
COLOUR-SCREEN  PLATES 

§ 44.  Choice  of  a Plate. — Whatever  make  of  colour 
plate  is  selected,  it  is  advisable  to  use  as  fresh  a packet 
as  possible. 

Messrs.  Lumiere  now  pack  the  plates  face  to  face 
between  black  cards,  and  guarantee  the  plates  good  for 
at  least  six  months  after  the  date  stamped  on  the  box. 
My  experience  in  South  Africa  does  not  coincide  with 
this.  I find  that  plates  begin  to  deteriorate  even 
before  the  date  stamped  on  the  box  has  expired.  This 
remark  applies  to  all  single  colour  plates.  I have 
shown  in  another  place  (§  73)  how  stale  plates  may 
be  revived.  Autochrome  and  Dufay  plates  are  now 
packed  in  boxes  bearing  the  date  up  to  which  they  may 
be  used,  and  it  would  be  well  if  other  makers  were  to 
adopt  the  same  course.  Plates  used  after  the  date 
stamped  on  the  box  are  found  to  be  less  sensitive, 
requiring  two  or  three  times  the  usual  exposure,  and 
moreover,  the  colours  are  somewhat  dull.  Still  older 
plates  show  other  marked  signs  of  deterioration,  and 
are  apt  to  become  veiled  with  fog  at  times. 

Each  make  of  colour-plates  has  certain  advantages 
and  certain  disadvantages,  and  the  reader  must  judge 


94 


PHOTOGRAPHY  IN  COLOURS 


for  himself  which  to  get.  We  may  say,  however,  that 
the  Autochrome  plate  gives  the  greatest  and  the  truest 
range  of  colours.  The  Dufay  and  the  Jougla  plates  are 
the  most  brilliant  in  the  colours,  while  the  Paget 
separate  plates  are  by  far  the  most  rapid.  The  latter 
make  of  plates,  however,  present  considerable  difficulties 
in  manipulation,  and  the  reader  must  not  be  surprised 
if  he  should  fail  in  getting  a good  result.  The  writer 
had  many  failures  before  he  finally  succeeded  in  pro- 
ducing a fairly  satisfactory  picture.  Lined  ruled  plates 
are  open  to  the  great  objection  that  they  form  a screen 
and  thus  give  rise  to  diffraction  spectra.  These  spectra 
are  very  noticeable  when  held  up  in  front  of  an  artificial 
light,  and  the  brighter  the  light  and  the  narrower  the 
rulings  the  more  conspicuous  are  the  spectra.  In  the 
Dufay  Dioptichrome  plate  the  lines  are  broken  up  by 
squares,  and  so  there  is  hardly  a trace  of  spectrum 
bands,  for  they  cannot  be  perceived  in  daylight,  but  in 
the  Warner-Powrie  positives  the  spectra  are  so  marked 
in  daylight  that  the  pictures  are  practically  worthless. 
In  the  Jougla  positives  the  spectra  are  very  conspicuous 
by  artificial  light,  but  are  only  slightly  noticeable  by 
daylight.  Of  course  the  Lumiere  plates  exhibit  no 
trace  of  spectra  since  there  is  only  a mosaic  of  dots. 
For  the  same  reason  the  Krayn  plates  are  free  from 
spectra,  but  the  squares  are  very  large,  and  hence  the 
definition  is  coarse. 

There  can  be  no  question  that  for  all-round  work 
the  Lumiere  plates  give  the  most  satisfaction  and  are 
the  easiest  to  manipulate,  but  the  Dufay  plates  are 
capable  of  producing  the  finest  results  of  any  plate  in 
the  market.  Some  of  the  positives  are  exquisite  in  their 


SINGLE  COLOUR-SCREEN  PLATES  95 


brilliancy,  richness  of  colour,  and  sharpness  of  outline, 
while  all  colours  except  blue  are  perfectly  rendered ; 
but  the  blues  have  much  too  green  a tinge,  especially 
the  skies.  On  the  other  hand,  red,  orange,  yellow  and 
greens  of  all  shades  are  superb  in  their  brilliancy  and 
delicacy  of  tones. 

§ 45.  Processes  concerned  in  making’  a Single 
Colour = Plate  Picture. — The  following  27  headings 
should  be  carefully  studied  by  the  Amateur  who  wishes 
to  obtain  a perfect  finished  picture  : — 

1.  The  camera. 

2.  The  lens. 

3.  Choosing  the  subject. 

4.  The  insertion  of  the  plate  in  the  slide. 

5.  The  colour  filter. 

6.  Focussing. 

7.  Use  of  a hood. 

8.  The  exposure. 

9.  The  dark-room  lamp. 

10.  Processes  concerned  in  the  formation  of  the 
colour  positive. 

11.  First  development. 

12.  Reversing  the  image. 

13.  Second  development. 

14.  Clearing  and  hardening. 

15.  Intensification. 

16.  Reduction. 

17.  Drying  the  positive. 

18.  Varnishing. 

19.  Covering  the  positive. 

20.  Final  improvements  of  the  tones  of  the  image. 

21.  Binding  up  the  colour  screen. 

22.  Defects  in  colour  positives,  causes  and  remedies. 


96  PHOTOGRAPHY  IN  COLOURS 


23.  Copying  of  colour  plates. 

24.  Indoor  colour- plate  portraiture. 

25.  Lantern  projections  in  natural  colours. 

26.  Resensitising  colour  screen  plates. 

27.  Repairing  light-filters  for  colour  photography. 

§ 45 — 1.  The  Camera. — Any  camera  which  takes 
plates,  and  not  films  only,  may  be  used,  since  a colour 
plate  is  exposed  exactly  in  the  same  way  as  any  ordinary 
plate,  the  only  difference  being  that  in  colour  photo- 
graphy a yellow  filter  is  essential,  whereas  in  ordinary 
photography  it  is  optional.  In  the  same  way  any  dark 
slide  may  be  used,  but  it  is  as  well  to  use  one  that  has 
plenty  of  room  inside,  for  it  must  be  borne  in  mind 
that  the  plate  is  reversed  (glass  side  towards  the  lens), 
and  as  the  film  is  exceptionally  thin,  and  liable  to  be 
scratched  or  torn,  nothing  should  be  allowed  to  touch 
it  except  velvet  or  smooth  tissue  paper.  If  the  makers 
would  only  construct  dark  slides,  so  that  the  film  side 
of  the  plate  merely  rested  along  the  edges  of  a rebate, 
enabling  the  surface  of  the  plate  merely  to  face,  without 
actually  touching,  the  partition  between  the  two  plates, 
it  would  prevent  those  flaws  and  green  spots  from 
appearing  after  development,  which  so  greatly  mar  the 
picture.  If  you  cannot  get  a slide  made  like  this,  it 
will  do  equally  well  to  protect  the  film  by  a frame  of 
cardboard  Jth  of  an  inch  wide,  on  which  the  edges  of  the 
plate  rest,  and  then  remove  the  spring  of  the  partition 
flap.  This  is  Grant’s  device,  and  answers  admirably 
if  the  slide  is  a folding  one.  If  you  have  a solid  dark 
slide,  such  as  Ross  and  many  others  provide,  it  is  a 
good  plan  either  to  cover  each  side  of  the  card  partition 
with  black  velvet,  or  to  gum  the  edges  of  two  or  three 


SINGLE  COLOUR-SCREEN  PLATES  97 


sheets  of  fine  tissue  paper  round  the  margins  of  the 
partition.  In  the  case  of  Paget  separate  plates,  the 
screen  and  sensitive  plate  must  be  kept  tightly  in 
contact  by  means  of  a strong  spring,  or  else  the  colours 
will  not  be  properly  formed  in  the  positive. 

§46—2.  The  Lens. — One  may  take  a picture  in  colour 
with  any  lens,  but  in  order  to  get  the  finest  definition, 
a lens  should  be  chosen  which  will  bring  not  only  the 
yellow,  green  and  blue,  but  the  orange  and  red  rays  to 
a common  focus.  In  photography,  with  ordinary 
plates  it  is  not  necessary  to  bring  the  orange  and  red 
rays  to  a focus,  since  the  film  is  not  affected  by  them, 
but  in  all  colour  work,  and  in  ordinary  photography 
when  isochromatic  or  panchromatic  plates  are  used, 
three  rays  must  be  brought  to  a focus,  viz.  the  red  and 
the  blue  and  the  yellow.  There  are  a large  number  of 
lenses  which  will  do  this  in  a satisfactory  manner,  in 
fact,  almost  any  anastigmat  lens  will  accomplish  it, 
especially  if  made  by  a first-class  firm.  At  the  present 
day  three  principal  types  of  lenses  are  in  common  use. 
The  Petzval  portrait  lens,  the  Aplanat,  which  is  a cheap 
but  very  efficient  lens  for  ordinary  photography,  and 
the  Anastigmat,  which  is  the  lens  par  excellence  for  all 
purposes.  It  is  a much  more  expensive  lens  than  the 
other  two,  but  owing  to  its  superb  definition,  wide 
aperture,  great  covering  power,  and  to  the  fact  that  all 
the  rays  of  the  visible  spectrum  are  practically  brought 
to  a common  focus,  it  forms  an  ideal  lens  for  colour 
photography.  Such  lenses  are  made  by  all  the  best 
firms — Zeiss,  Goerz,  Dallmeyer,  Ross,  Watson,  Cooke 
(Taylor  & Hobson),  Beck,  Stayley,  Aldis,  Voigtlander, 
Busch,  Lacour-Berthiot  (Paris),  Salmoiraghi  (Milan), 

11 


98  PHOTOGRAPHY  IN  COLOURS 

and  many  others.  The  reader  will  not  make  a mistake 
if  he  procures  an  Anastigmat  from  any  of  the  above- 
mentioned  firms. 

Which  is  the  best  focal  length  of  lens  to  use  ? For 
ordinary  photography  a lens  having  a focal  length 
equal  to  the  diagonal  of  the  plate  is  generally  recom- 
mended. Thus  for  a quarter-plate  a 5J  in.  or  6 in. 
lens,  for  a 5x4  plate,  a 6J  in.  or  7 in.  lens,  and 
for  a half -plate  an  8-in.  lens  will  be  found  to  give 
the  best  all-round  results.  In  colour  photography, 
however,  a lens  having  a much  longer  focal  length  will 
be  found  to  give  more  artistic  results.  In  fact,  the 
lens  suitable  for  the  next  larger  size  of  plate  should  be 
used.  Professor  Miethe  goes  even  further,  and  recom- 
mends a lens  of  14  to  17  cm.  (5|  in.  to  7 in.)  for  a 
lantern  slide,  one  of  7 in.  to  8 in.  for  a quarter-plate, 
one  of  8 in.  or  9 in.  for  a 5 X 4,  and  one  of  10  in. 
or  12  in.  focal  length  for  a half-plate.  The  reasons  for 
this  are  several.  First,  there  is  always  a danger  of 
overcrowding  too  much  on  the  plate.  With  a black 
and  white  picture  this  does  not  matter  so  much,  as  it 
can  always  be  enlarged,  and  this  is  always  done  if  the 
photographer  intends  to  put  a lot  of  work  on  the  print 
afterwards.  With  a colour  positive,  however,  an 
enlargement  is  not  only  a more  difficult  and  tedious 
business,  but  the  picture  loses  a great  deal  more  by 
enlargement  than  an  ordinary  negative,  for  reasons 
which  we  have  given  elsewhere.  Secondly,  what  one 
wants  in  a colour  picture  are  large  surfaces  of  one 
colour,  rather  than  a number  of  tiny  patches  of  different 
colours.  With  a short  focus  lens  this  is  generally  the 
case,  since  the  magnification  is  so  small,  and  one  is 


SINGLE  COLOUR-SCREEN  PLATES  99 


apt  to  get  a profusion  of  small  coloured  objects  which 
carry  the  eye  all  over  the  picture,  instead  of  the  eye 
being  arrested  by  the  principal  subject,  as  it  ought  to 
be,  and  thus  the  general  effect  is  spoilt.  Again,  with 
a short  focus  lens  the  background  will  appear  too 
small  in  comparison  with  the  foreground,  an  effect 
which  will  always  destroy  the  balance  and  harmony  of 
the  whole.  Of  course  these  remarks  apply  equally  well 
to  every  kind  of  colour  photography. 

§ 47 — 3.  Choosing  the  Subject. — In  choosing  a 
subject,  avoid  too  much  contrast  in  the  lighting.  In 
ordinary  photography  deep  shadows  and  brilliant  high- 
lights often  contribute  largely  towards  forming  a 
harmonious  picture,  but  in  colour  photography  extremes 
of  light  and  shade  when  in  large  masses  and  in  the 
same  picture  are  very  difficult  to  develop  properly,  the 
reason  being  that  the  high-lights  have  to  be  greatly 
over-exposed  to  bring  out  detail  in  the  dark  parts 
and  shadows.  Now,  as  we  shall  see,  a greatly  over- 
exposed subject  will  be  eaten  away  in  the  reversing 
bath,  and  leave  a thin  detail-less  picture,  while  the 
under-exposed  dark  subject  will  be  dull,  heavy,  and 
opaque.  For  example,  if  a portrait  group  be  taken 
against  a very  dark  laurel  or  holly  bush,  the  flesh  tints, 
and  white  costumes  especially,  will  appear  washed-out 
and  thin  after  reversal,  if  development  be  carried  far 
enough  to  bring  out  the  details  of  the  dark  leaves.  On 
the  other  hand,  if  the  figures  be  rightly  developed  the 
leaves  will  come  out  dense  and  without  detail  in  the 
reversing  bath.  Of  course,  in  many  cases,  this  latter 
adds  to  the  pictorial  effect.  If,  however,  detail  every- 
where is  wanted,  the  only  thing  to  be  done  is  to  take 


IOO 


PHOTOGRAPHY  IN  COLOURS 


the  portrait  over  again,  either  with  a much  lighter  back- 
ground, or  if  that  is  impossible,  by  using  a concentrated 
developer  to  secure  detail  in  the  shadows,  and  long 
before  the  development  is  finished  to  pour  it  off, 
rinse  the  plate,  and  renew  development  with  a dilute 
developer,  so  as  to  allow  the  image  to  become 
strengthened  very  slowly.  In  the  same  way,  it  is 
almost  impossible  to  photograph  the  interior  of  a 
church  and  at  the  same  time  get  good  colours  in  the 
windows.  This  can  only  be  done  either  by  using  the 
modified  developer  just  mentioned,  or  else  by  brushing 
over  the  windows,  as  soon  as  development  has  begun, 
with  a solution  of  Bromide  of  Potassium,  so  as  to 
restrain  the  action  of  the  developer  locally.  Above 
all,  it  is  essential  to  give  the  correct  exposure  for  the 
dark  parts  and  shadows. 

The  correct  exposure  is  the  key  to  the 
whole  position,  and  the  greater  the  con- 
trasts the  more  necessary  it  becomes  to 
get  it  absolutely  right.  Then  the  modifica- 
tions in  development  above  described  will  secure  the 
desired  result. 

Groups  and  portraits  often  come  out  better  if  taken 
when  fully  illuminated  by  sunshine.  This,  as  the 
reader  is  well  aware,  is  not  the  case  with  ordinary 
photography,  but  in  colour  work,  the  colours  come  out 
much  more  brilliantly  in  full  sunlight,  and  the  result 
is  not  a flat  picture,  as  one  would  expect,  but  often  a 
most  pleasing  effect  is  thereby  attained  without  any 
appearance  of  flatness. 

§ 48 — 4.  Insertion  of  the  Plate  into  the  Slide. — 

The  plate  should  be  inserted  into  the  slide  several  feet 


SINGLE  COLOUR-SCREEN  PLATES  IOI 


away  from  a very  feeble  yellow-green  or  “ Virida  ” light 
(see  § 53),  taking  care  to  have  the  glass  surface  facing 
the  lens,  in  order  that  the  light  may  pass  through  the 
starch  grain  filter  before  reaching  the  film.  The  bright 
reflection  of  the  lamp  will  show  at  once  which  is  the 
glass  side,  as  the  coated  side  does  not  reflect  the  light 
at  all.  Most  plates  are  packed  film  to  film,  with  two 
pieces  of  thin  brownish  paper  between  them.  These 
need  not  be  removed,  but  a black  card 1 should  be  placed 
on  the  top  of  the  paper,  and  then  the  plate  covered 
with  both,  should  be  placed  in  the  slide.  In  this  way 
the  film  is  doubly  protected  both  from  dust  and 
friction,  as  well  as  from  any  stray  light.  A better  plan 
still  is  to  have  the  slides  reconstructed  so  that  the 
whole  of  the  film  except  the  extreme  edges  rests  against 
air  only,  since  the  card,  pressed  by  the  spring,  may  act 
on  the  plate  injuriously.  This  prevents  contact  of  the 
film,  except  along  the  edges,  so  that  there  is  no  chance 
of  abrasions  whereby  the  water  may  soak  in  and  give 
rise  to  green  or  other  pigment  stains  during  develop- 
ment. 

§ 49 — 5.  The  Colour  - Filters. — In  order  to  give  the 
correct  values  to  the  three  colours  during  exposure, 
it  is  imperative  to  put  a colour-filter  either  in  front  of 
or  behind  the  objective,  so  as  to  reduce  the  excessive 

1 A white  card  will  do  just  as  well,  and  if  placed  in  contact 
with  the  film  will  reduce  the  exposure  to  about  fths,  i.e.  if  using 
no  card  at  all,  or  a black  card,  the  exposure  required  is  10 
seconds,  with  a white  glazed  card  the  exposure  will  he  6 seconds. 
Be  careful  not  to  expose  the  glazed  card  to  bright  sunlight  as 
the  glaze  is  often  fluorescent  and  the  luminous  exhalations  are 
apt  to  fog  the  plate  slightly.  Interference  phenomena  have  no 
effect  on  the  image. 


102 


PHOTOGRAPHY  IN  COLOURS 


action  of  the  bine  end  of  the  spectrum.  If  no  filter 
at  all  is  used,  or  unfiltered  white  light  gets  access 
to  the  plate,  the  final  image  will  appear  throughout 
of  a violet-blue  colour.  Moreover,  any  ordinary  yellow 
screen  will  not  give  correct  values ; in  fact,  the  attain- 
ment of  the  correct  colour  is  one  of  the  difficulties 
which  all  makers  of  screen-plates  have  had  to  overcome, 
since  not  only  must  the  ultra-violet  rays  be  absorbed, 
but  the  correct  proportion  of  the  spectrum  colours  must 
be  arrived  at,  so  as  to  get  the  right  balance  of  colours. 
The  filter  which  Lumiere  finally  adopted  (auto-filter) 
consists  of  a piece  of  glass  coated  with  gelatine,  and 
stained  a delicate  rose  orange-yellow  colour,  and  pro- 
tected by  a second  piece  of  optically  worked  glass.  The 
Lumiere  filters  are  issued  in  five  sizes,  in  a square 
form.  This  is  an  awkward  shape,  as  it  takes  up  too 
much  room  and  requires  a box  adaptor  to  attach  it  to 
the  lens.  Both  Boss  and  Sanger  Shepherd  & Co.  under- 
take to  trim  and  grind  these  square  screens  to  circles, 
which  is  the  most  convenient  form,  as  they  can  be 
fitted  either  into  split  rings  or  solid  rings,  which  screw 
on  to  the  flange  of  the  lens,  or  let  into  the  lens  cap  with 
the  front  removed.  These  latter  methods  allow  of  the 
filter  being  left  on  the  lens,  and  one  runs  no  risk  of 
leaving  it  behind,  or  forgetting  to  put  it  on  before  ex- 
posure. It  is  quite  easy  to  shape  the  square  to  a circle 
one’s  self.  It  is  only  necessary  to  mark  the  circle  in 
ink  on  one  of  the  surfaces,  and  then  crunch  off  small 
particles  of  glass  through  both  thicknesses  at  once  with 
an  optician’s  edging  shanks,  until  the  circle  is  reached. 
A couple  of  minutes’  work  on  a grindstone  will  round 
it  off  to  a smooth  edge.  The  glass  will  break  if  a 


SINGLE  COLOUR-SCREEN  PLATES  103 


diamond  be  used,  because  the  two  faces  are  cemented 
together. 

Wratten’s  K1  Filter  does  fairly  well  for  Thames 
plates,  but  is  useless  for  Autochromes.  It  is,  how- 
ever, necessary  to  supplement  the  ordinary  filter  when 
exposing  upon  snow  or  ice  by  a second  filter,  owing 
to  the  great  excess  of  blue-violet  light  in  these  cases. 

When  copying  oil-paintings,  especially  in  a gallery, 
a lighter  tint  filter  gives  better  results  than  Lumiere’s 
auto-filter,  which  we  have  referred  to  above.  Lumiere 
supplies  one  under  the  name  of  “ Auto-JM  filter.” 

§ 50 — 6.  Focussing1. — As  the  film  surface  of  the  plate 
is  turned  away  from  the  lens,  it  is  advisable  to  reverse 
the  focussing  ground  glass  (or  both  ground  glasses  in  a 
reflex  camera).  Then  the  plane  of  the  film  will  corre- 
spond with  that  of  the  image  surface  of  the  ground  glass, 
and  whether  you  put  the  colour-filter  in  front  or  behind 
the  lens  it  will  make  no  difference,  because  the  correc- 
tion due  to  the  filter  is  made  with  the  eye  when  focus- 
sing. If  no  ground  glass  is  used,  and  the  focus  adjusted 
by  the  scale,  when  the  filter  is  placed  behind  the  lens 
this  will  just  correct  the  error,  because  the  filter  is 
usually  3 mm.  thick,  and  the  colour  plate  T5  mm. 
Now,  the  effect  of  a filter  behind  a lens  is  to  lengthen 
out  the  focus  one-third  of  the  thickness  of  the  filter. 
But  in  this  case  the  light  passes  through  two  plates  of 
glass  (viz.  the  colour  filter  3 mm.  thick  and  the  sensi- 
tive plate  1*  5 mm.  thick),  the  displacement  will  therefore 
be  one-third  of  (3  mm.  -f-  1*5  mm.),  or  1*5  mm.  behind 
the  front  (or  glass)  surface  of  the  plate.  Therefore,  this 
displacement  of  the  focus  just  coincides  with  the  thick- 
ness of  the  sensitive  plate  (since  it  is  reversed  and  the 


104  P HO T 0 GRA PH Y IN  COLOURS 


film  is  now  behind),  and  no  adjustment  will  be  needed, 
unless  a thinner  plate  be  used,  say  of  1 mm.  thick,  when 
it  will  still  be  necessary  to  rack  out  about  O’ 3 mm.  If 
the  filter  is  in  front  of  the  lens  it  will  have  no  effect  on 
the  focus  if  the  object  is  at  some  distance,  i.e.  if  the 
lens  is  in  focus  near  the  infinity  mark.  In  this  case 
the  lens  must  be  racked  in  an  amount  equal  to  the 
thickness  of  the  sensitive  plate  (1-5  mm.).  If  very  small- 
sized plates  are  used,  a movement  of  1 mm.  in  will  suffice. 

§ 51 — 7.  Use  of  a Hood  and  Effect  of  Shadows 
from  Coloured  Objects. — It  will  often  be  found  ad- 
vantageous to  use  some  form  of  hood  in  front  of  the 
lens,  so  as  to  screen  off  all  the  light  not  actually  used  in 
forming  the  picture.  For  this  purpose  one  with  an 
oblong  opening,  the  size  and  shape  of  the  picture,  gives 
the  best  results,  as  all  rays  are  cut  off  which  do  not  enter 
into  forming  the  picture,  and  which  would  interfere 
with  the  purity  of  the  image.  In  ordinary  work  this 
is  of  little  consequence,  but  in  colour  work  all  reflected 
light  spoils  the  purity  of  the  colours,  either  by  shedding 
white  light  (which  is  a mixture  of  all  the  colours),  or 
one  particular  colour,  on  to  them.  Thus,  a green 
object  casting  a shadow  on  a red  object  will  give  rise  to 
an  orange-yellow,  or  a bright  silver  teapot  on  a red 
tablecloth  will  appear  splashed  with  red  or  pink  owing 
to  the  red  light  reflected  from  the  cloth  on  to  the 
teapot.  Hence  in  portraiture  it  is  not  uncommon  to 
see  the  neck  of  the  sitter  appear  of  a greenish-yellow 
colour  owing  to  the  reflected  light  from  an  adjacent 
mass  of  green  leaves  being  thrown  across  it,  but  it 
is  only  right  to  add  that  under-exposure  is  often 
responsible  for  this.  Another  point  is  to  regulate  the 


SINGLE  COLOUR-SCREEN  PLATES  105 


exposure  so  as  to  give  the  correct  exposure  to  both  sky 
and  foreground  in  the  picture.  This  can  be  done  by 
means  of  a flap-shutter,  a guillotine  (up  and  down) 
shutter,  or  a black  card  moved  up  down  by  the  hand. 
You  may  get  a blue  sky  and  clouds  with  correctly 
exposed  landscape  in  several  ways.  First,  you  may 
give  a shorter  exposure  to  the  sky,  by  means  of  a 
flap-shutter  or  a screen.  Secondly,  you  may  over- 
expose the  whole  subject  about  twice  and  considerably 
shorten  the  time  of  development.  Or,  lastly,  you  may 
lift  the  plate  out  of  the  developer  and  then  pour  a little 
water  containing  1 % Bromide  of  Potassium  over 
the  sky  area  of  the  plate,  returning  it  immediately 
to  the  developer,  repeating  the  process  three  or  four 
times  if  necessary.  In  fact,  generally  speaking,  the 
plate  may  be  treated,  under  special  circumstances,  in 
much  the  same  manner  as  in  dealing  with  a similar 
subject  on  an  ordinary  plate. 

§ 52 — 8.  The  Exposure, — The  right  exposure  is  a 
matter  of  great  difficulty.  This  is  the  more  to  be  de- 
plored as  the  success  of  thefinishedpicture 
depends  largely  on  the  correct  exposure. 
This  is  so,  to  a far  greater  extent  than  is  the  case  with 
ordinary  plates.  As  a rough  guide,  it  may  be  said 
that  one  should  under-expose  for  subjects 
taken  in  sunlight  and  at  midday  in  sum- 
mer, and  considerably  over-expose  (two  or  two 
and  a half  times  the  calculated  time)  in 
dull  light,  or  towards  evening,  or  for 
objects  in  the  shade.  You  may  use  either 
Wynne’s  or  Watkins’  or  the  Imperial  plate  Actino- 
meter,  which  is  calculated  on  Hurter  and  Driffield’s 


io6 


P HO T O GRA PH Y IN  COLOURS 


numbers,1  or  else  Wellcome’s  exposure  record.  The 
latter  gives  you  the  Autochrome  plate  speed  with 
colour-filter  Indoors  as  24,  Outdoors  12,  which  speed 
we  may  compare  with  Imperial  Special  Rapid  or  Flash- 
light. The  Autochrome  filter  increases  the  exposure 
about  five  times;  the  starch  grain  backing  about  six 
times,  while  the  film  is  about  the  same  speed  as  an 
Imperial  Rapid.  If,  therefore,  you  take  an  Ilford 
“ Zenith,”  an  Imperial  “ Sovereign  ” plate,  or  a 
“ Premo  Filmpack,”  and  use  any  one  of  them  with- 
out a filter  in  a good  light  and  give  minutes  for 
seconds,  you  will  get  about  the  right  exposure  under 
similar  conditions  for  a Lumiere  Autochrome  plate 
with  filter.  Whatever  you  do,  beware  of  under-ex- 
posure. An  under-exposed  plate  can  never  yield  a 
perfect  picture ; an  over-exposed  plate,  even  up  to  three 
times  the  correct  exposure,  can  be  turned  into  a splen- 
did positive  if  carefully  restrained  during  development 
by  bromide  of  potassium,  and  by  diluting  the  developer 
with  water.  In  photographing  sunsets  rather  under- 
develop than  overdevelop ; overdevelopment  ruins 
sunset  effects. 

§ 53 — 9.  The  Dark=room  Lamp  or  Safelight. — 

Since  the  film  of  an  Autochrome,  Omnicolore,  or  any 
other  three-colour  plate,  must  of  necessity  be  a Pan- 
chromatic one,  it  is  obvious  that  the  same  light  should 
be  used  for  all  red-sensitive  plates.  Formerly  the  writer 

1 H.  and  D.  is  equivalent  to  1£  Watkins,  so  that  to  convert  H. 
and  D.  into  Watkins,  add  one-half  ; or  take  one-third  off  Watkins 
to  obtain  H.  and  D.  equivalent.  See  Appendix,  Table  1 for  correct 
exposures.  For  Autochromes  out  of  doors  use  H.  and  D.  2, 
Watkins’  Meter  3,  Wynne’s  Meter  11.  For  Paget  plates  use  H. 
and  D.  5,  Watkins’  3,  and  Wynne’s  18. 


SINGLE  COLOUR-SCREEN  PLATES  107 


used  a very  suppressed  dark  red  safelight,  consisting 
of  two  sheets  of  ruby  glass,  one  sheet  of  orange-red 
wrapping  paper,  and  two  of  canary  yellow  fabric,  but 
a bluish-yellow-green  light  is  unquestionably  safer  and 
less  irritating  to  the  eye  than  a deep  ruby  red.  Be- 
sides, as  Purkinje  first  pointed  out,  blue  and  blue-green 
colours  are  more  readily  perceived  in  a dim  light  than 
red  or  yellow  (see  § 25).  An  admirably  safe  coloured 
paper  is  now  sold,  in  packets  containing  yellow  and 
green  sheets,  by  Messrs.  Lumiere,  under  the  name  of 
“ Yirida  ” Paper.  For  use  take  three  yellow,  and  two 
bluish-green  papers.  Cut  them  to  the  size  of  the 
lantern  window  and  place  all  six  together  between  two 
sheets  of  plain  glass,  and  slide  them  into  the  groove  of 
the  lamp  in  the  place  of  the  usual  red  filter.  The  three 
yellow  papers  are  recommended  to  be  placed  next  the 
light.  The  writer  uses  an  extra  bluish-green  paper 
gummed  on  to  a plate  of  glass,  which  he  places  in  front 
of  the  lantern  until  development  is  half  completed. 
A developing  lamp  is  now  issued  by  Messrs.  Wratten 
and  Wainwright  which  has  some  excellent  features. 
It  is  so  constructed  that  only  diffused  light  is  seen,  the 
direct  rays  from  the  gaslight  or  lamp  being  entirely 
blocked  out,  but  those  rays  which  spread  backwards 
are  reflected  by  a bent  sheet  of  white  enamelled  metal 
through  the  yellow-green  screen.  Thus  the  light  is 
not  only  safer,  but  is  much  softer  and  more  uniformly 
distributed.  Wratten’s  screen  consists  of  a sheet  of 
glass  coated  with  a bright  yellow  gelatine  film,  one 
coated  with  a bright  green  film,  and  a thick  sheet  of 
specially  tested  green  paper  between  the  two. 

Some  operators  soak  the  plate  in  the  developer  for 


io8 


PHOTOGRAPHY  IN  COLOURS 


two  minutes  in  total  darkness,  before  adding  the  alkali 
accelerator,  which  starts  the  development.  This  de- 
sensitises the  plate  sufficiently  to  allow  of  an  ordinary 
deep  red  light  being  used.  Others  soak  it  in  a 2 % 
solution  of  soda  bisulphite  or  metabisulphite  to  which 
a little  Bromide  of  Potassium  has  been  added.  This 
effects  the  same  purpose. 

§ 54 — 10.  Processes  concerned  in  the  Forma- 
tion of  the  Coloured  Positive. — The  treatment  of  the 
exposed  plate,  in  order  to  obtain  the  complete  picture, 
appears  complicated,  and  is  apt  to  frighten  the  beginner, 
but  it  is  really  an  exceedingly  simple  process,  and  only 
occupies  about  fifteen  minutes  from  the  time  the 
plate  is  put  into  the  first  developer  to  the  completion 
of  the  positive.  If  you  omit  intensification,  clearing, 
and  varnishing  (none  of  which  are  essential),  everything 
necessary  can  be  done  with  two  solutions,  viz.  a 
developer  and  a reversing  solution,  as  the  same  bath 
serves  for  both  first  and  second  development. 

§ 55 — 11.  First  Development. — This,  as  well  as  all 
subsequent  operations,  excepting  the  reversal  process, 
are  exactly  the  same  as  with  other  plates.  Pyro- 
ammonia,  Pyro-soda,  Metoquinone,  Quinomet,  and 
Eodinal  all  give  excellent  results. 

The  chief  thing  to  remember  is  that  when  working 
with  Autochromes  you  must  cut  down  the  time  of  the 
various  washings  as  much  as  possible,  as,  owing  to  the 
extreme  thinness  and  delicacy  of  the  film,  it  will  not 
stand  long  immersion  in  any  liquid,  nor  will  the  film 
stand  a jet  of  water,  such  as  would  have  no  effect 
whatever  on  an  ordinary  negative  film.  With  other 
colour  plates  it  does  not  so  much  matter.  The 


SINGLE  COLOUR-SCREEN  PLATES  109 


following  method,  recommended  by  Lumiere,  is  reliable, 
and  if  the  tyro  follows  it  carefully  be  will  certainly 
correct  bis  under-  or  over-exposures  very  materially. 

§ 56.  Rules  for  Development. — Clean  two  white 
porcelain  or  glass  dishes.  See  that  the  water  and 
developer  are  of  the  right  temperature,  between  55°  F. 
and  65°  F.  This  is  of  the  utmost  importance.  If  it  is 
too  cold  you  will  not  get  the  image  out  properly,  if  it 
is  too  warm  you  will  get  frilling  and  over- development. 

For  a quarter-plate  or  9 X 12  cm.  plate  put  40  c.c. 
(1  oz.  2 drms.)  of  distilled  water  (by  preference)  into  a 
cup.  Add  to  it  2*5  c.c.  (42  m.)  of  concentrated  Quino- 
met  developer.  Place  it  by  the  lamp. 

Into  a second  smaller  measure  put  7*5  c.c.  (2  drms. 
8 m.)  of  same  developer.  Place  it  next  to  the  other 
measure. 

Adjust  your  watch  or  clock  so  that  the  minute  hand 
is  at  a full  minute  when  the  second  hand  reaches 
zero.  Put  the  watch  in  the  best  light  possible.  Place 
the  Lumiere  development  time-table  near  the  lamp,  or, 
better  still,  write  it  in  Indian  ink  on  the  outermost 
paper  of  the  lamp  (see  Appendix,  Table  9,  p.  272). 

In  a separate  dish  place  the  Acid  Permanganate 
or  Acid  Bichromate  solution.  Put  it  anywhere  away 
from  the  lamp.  Place  the  dark  slide  in  a dry  place 
away  from  the  light.  Light  the  green  lamp,  let  your 
eye  get  accustomed  to  the  green  light,  and  then  note 
the  position  of  everything  you  want  before  making  the 
room  dark. 

When  all  is  ready  remove  the  plate  from  the 
slide  carefully.  Put  the  plate  film  side  (lighter  side) 
upwards  in  the  dish,  in  almost  total  darkness , and  holding 


I IO 


PHOTOGRAPHY  IN  COLOURS 


it  in  shadow  near  the  lamp ; wait  until  the  second 
hand  is  about  to  reach  zero,  noting  at  the  same  time 
the  position  of  the  minute  hand.  Then  pour  the 
developer  over  one  end  of  the  plate,  cover  the  dish 
with  a card,  and  rock  well,  so  as  to  cover  up  instantly 
any  islands  of  film  that  may  form.  After  12  seconds 
hold  the  dish  nearly  at  right  angles  to  the  light,  so 
as  not  to  allow  much  light  to  fall  on  the  surface, 
uncover  the  dish  and  watch  for  the  first  indications  of 
an  image  other  than  sky.  The  moment  the  image 
begins  to  be  visible  you  can  let  more  light  fall  on  the 
plate.  Then  note  the  position  of  the  second  hand,  and 
immediately  pour  the  7*5  c.c.  Quinomet  solution  over 
the  plate  while  rocking.  Put  a cover  over  the  dish  and 
gently  rock  the  dish  until  the  time  is  up  according  to 
the  time-table.  If  the  image  fails  to  appear  after  40 
seconds,  add  22  c.c.  of  Quinomet.  If  no  image  appears 
at  the  end  of  60  seconds,  it  will  be  hopelessly 
under-exposed.  The  only  chance  is  to  add  an  ounce 
of  water  and  leave  it  for  another  minute.  If  the 
image  begins  to  appear,  prepare  some  fresh  developer 
(33  c.c.  of  Quinomet  to  1J  oz.  of  water),  cover  up 
the  dish  and  wait  until  the  image  is  dark  enough, 
examining  it  for  an  instant  from  time  to  time,  until, 
on  holding  the  plate  horizontally  against  the  light,  the 
details  of  the  subject  can  be  clearly  perceived.  These 
details  need  not  be  nearly  as  dark  as 
they  should  be  in  an  ordinary  nega- 
tive. Then  wash  well  in  a dish,  pouring  the  water 
off  and  filling  up  again  about  five  times. 

If  you  prefer  a Sodo-Pyro  or  Hydroquinone  and 
Metol  developer  you  can  pour  over  the  full  strength 


SINGLE  COLOUR-SCREEN  PLATES  III 


solution  right  away,  watch  for  the  appearance  of  the 
first  trace  of  image,  neglecting  the  sky,  multiply  the 
time  by  the  factor  number,  and  cover  up  the  dish  with 
a card  until  the  calculated  time  is  up,  then  rinse  well 
in  the  dark  and  place  in  several  baths.  In  developing 
the  plate  there  are  three  marked  stages.  First,  a 
gradual  strengthening  of  the  image  until  it  reaches 
a certain  maximum ; second,  the  image  gradually 
loses  strength  until  it  has  almost  disappeared,  owing 
to  the  loss  of  opacity  of  the  white  emulsion.  The 
darker  parts  of  the  image  remain,  but  they  appear 
transparent ; third,  if  the  development  be  pushed  be- 
yond this  stage,  the  image  in  the  clear  parts  gradually 
change  into  a positive.  It  is  at  the  second  or  trans- 
parent stage  when  development  must  be  stopped  to 
get  the  finest  result. 

§ 57 — 12.  Reversal  of  Image. — The  negative  (still  in 
the  dark)  is  now  put  in  a clean  dish  and  the  acid  per- 
manganate solution  (or  acid  bichromate  solution) 
poured  over.  Eock  well  for  a few  seconds.  Cover 
up  the  dish  for  half  a minute  and  then  turn  up  the 
gaslight.  After  three  or  four  minutes  remove  the 
negative  (now  a positive)  and  wash  in  four  or  five 
quick  changes  of  water  as  before.  The  image  should 
be  examined  from  time  to  time  in  daylight  or  gaslight, 
and  reversal  stopped  the  moment  all  the  details  are 
fully  out,  otherwise  the  high  lights  will  be  eaten  away 
and  detail  lost  in  them. 

§ 58 — 13.  Second  Development. — Expose  the  posi- 
tive to  bright  daylight  for  a few  seconds,  or,  if  at  night, 
burn  a foot  or  two  of  magnesium  six  inches  from  the  film, 
holding  the  plate  upright  to  avoid  any  ash  falling  on 


I 12 


PHOTOGRAPHY  IN  COLOURS 


the  plate,  or  hold  it  close  to  an  incandescent  burner  or 
Osram  lamp  for  a minute  or  two.  (If  you  have  no 
ribbon,  or  are  called  away,  shake  off  the  superfluous 
water  and  leave  it  to  dry  anywhere  in  subdued  light. 
It  may  then  be  developed  at  your  leisure.)  Then  put 
the  plate  in  the  first  developing  bath  in  bright  daylight 
and  leave  it  there  until  the  positive  becomes  uniformly 
dark,  in  fact  nearly  or  quite  black,  by  reflected  light, 
which  occurs  after  three  or  four  minutes.  This  process 
must  be  very  thorough  if  intensification  is  required 
afterwards,  because  the  necessary  hypo  bath  afterwards 
will  cause  the  image  to  fade  if  the  second  development 
is  incomplete.  Einse  well  under  the  tap. 

§ 59 — 14.  Clearing. — If  the  image  looks  dull  and 
somewhat  brownish  it  may  be  cleared  and  rendered 
bright  by  soaking  in  a weak  1 % solution  of  Sodium  Bi- 
sulphite, or  the  old  reversal  solution  diluted  to  a pale 
colour,  about  one  part  to  20  or  30  of  water.  The 
plate  should  not  be  left  in  the  bath  more  than  30 
seconds. 

§ 60 — 14a.  Hardening. — This  is  optional,  but  useful, 
as  it  brightens  up  the  picture  and  hardens  the  film.  Put 
the  plate  in  a bath,  made  by  dissolving  about  half  an 
ounce  of  chrome  alum  to  a quart  of  tap  water.  It  may 
be  used  over  again  a great  many  times,  or  you  may 
use  1 part  of  alum,  2 parts  citric  acid,  and  100  parts 
of  tap- water.  This  is  a favourite  bath  of  the  writer’s  ; 
it  may  be  used  over  and  over  again.  Formalin  solution, 
1 /,  is  also  recommended.  It  is  essential  to  use  one 
of  them  in  hot  weather,  or  if  the  temperature  of  the 
water  or  developer  exceeds  65°  F. 

Wash  the  positive  in  four  or  five  quick  changes  of 


SINGLE  COLOUR-SCREEN  PLATES  1 1 3 

water  (3  or  4 secs,  between  each  change).  Shake  off 
superfluous  water  and  let  it  dry  in  an  upright  position. 
The  positive  is  now  finished,  provided  it  has  been 
correctly  exposed  and  developed. 

§ 61 — 15.  Intensification. — Often  the  colours  are 
not  bright  enough,  or  they  are  too  thin  and  watery,  or 
lastly,  they  are  dull,  heavy  and  dense.  For  the  two 
former  cases  intensification  will  probably  put  matters 
right.  In  the  latter  case  reduction  will  increase  the 
transparency  and  brightness,  giving  the  transparency 
more  “ pluck.”  It  must  then  be  intensified  to  bring  up 
the  colours.  We  will  first  give  the  explanation  and 
then  the  practice  of  Intensification. 

§ 62.  Explanation  of  the  Process  of  Intensifica- 
tion.— If  the  positive  looks  weak  and  the  colours 
are  not  as  bright  as  they  should  be,  it  should 
be  intensified.  The  following  diagram  shows  what 
happens.  We  have  seen  that  in  the  positive  the  com- 
plementary colours  are  hidden  behind  a deposit  of 
blackened  silver.  When  the  positive  looks  weak  the 
deposit  is  too  thin,  and  the  contrast  between  the  trans- 
parent parts  and  the  veiled  parts  is  not  sufficiently 
prominent.  If,  therefore,  we  intensify  the  deposit  by 
adding  another  coat  of  blackened  deposit  to  it,  the 
contrast  between  the  two  will  evidently  become  more 
vivid.  This,  in  a word,  is  the  rationale  of  intensification. 

Suppose  a bright  orange  object  which  consists  of  a 
full  measure  of  red,  some  green  and  a trace  of  blue,  be 
photographed,  the  positive,  if  correctly  exposed  in  the 
first  instance,  will  show  in  section  an  appearance 
similar  to  Fig.  18.  Here  all  the  bromide  of  silver 
behind  the  red  grains,  about  half  the  green  and  a 

1 


PHOTOGRAPHY  IN  COLOURS 


114 

quarter  of  the  blue,  will  have  been  acted  upon  and 
dissolved.  If  the  plate  is  under-exposed,  the  proportions 
will  be  altered.  Only  half  the  silver  bromide  behind 
the  red,  a quarter  of  the  green,  and  a mere  trace  of 
the  blue  (Fig.  18)  will  be  reduced. 

If  now  we  reduce  this  deposit  by  a dilute  solution  of 
acid  permanganate  we  can  take  a layer  of  deposit  off 
the  whole  of  the  film,  so  that  we  may  have  about  three- 


before  after 

'v. y 

V 

Intensification 


Exposure 

A.  B.  C.  D. 

Fig.  18. 


quarters  clear  glass  behind  the  red,  nearly  half  behind 
the  green,  and  a quarter  behind  the  blue.  In  this  way 
we  can  improve  the  picture.  If  the  plate  be  over- 
exposed, it  is  obvious  that  a much  larger  quantity  of 
bromide  of  silver  would  be  acted  on  by  the  light  and 
reduced  to  silver  by  the  developer,  and  as  this  would 
all  be  dissolved  away  by  the  acid  permanganate,  we 


SINGLE  COLOUR-SCREEN  PLATES  I15 

should  get  nearly  clear  glass  behind  the  red  and  green, 
so  that  there  would  be  very  little  left  to  redevelop,  and 
a section  would  resemble  C (Mg.  18).  If  this  layer  be 
intensified  by  any  method,  we  should  again  get  an 
appearance  more  like  D by  increasing  the  deposit  over 
the  green,  but  much  more  in  proportion  over  the  blue. 
In  this  way  the  contrasts  will  be  heightened  and  the 
colours  made  more  brilliant,  since  the  blue  which  is 
the  opposing  colour  is  nearly  blocked  out,  and  so 
allows  the  red  and  green  to  shine  unopposed  by 
their  complementary  blue.  As,  however,  the  deposits 
behind  the  red  would  also  be  increased,  whereas  in  the 
correctly  exposed  positive  there  was  none  at  all,  we 
should  change  a dull  yellow  into  a brilliant  yellow 
instead  of  a brilliant  orange,  as  it  ought  to  be.  This 
also  shows  us  how  the  slightest  error  in  any  one  colour 
will  always  upset  the  proper  proportions  of  the  other 
colours,  which  will  be  still  more  disturbed  on  any 
attempt  to  freshen  up  the  colours  by  intensification  or 
reduction,  although  either  may  greatly  increase  the 
brilliancy  and  beauty  of  the  image. 

In  the  same  way  any  alteration  in  the  intensity  of 
illumination  of  the  object  will  modify  the  colours, 
since  we  have  reasons  for  believing  that  Purkinje’s 
Phenomenon  holds  good  for  the  plate  as  well  as  for 
the  eye.  On  the  other  hand,  if  the  total  amount  of 
light  reaching  the  plate  is  the  same  in  two  cases,  no 
matter  how  it  is  portioned  out,  the  result  will  be  the 
same.  Thus,  if  you  photograph  a well-lighted  grey- 
coloured  object  with  a lens  working  at  F/4,  exposing 
it  for  two  seconds,  and  photograph  again  with  the  lens 
stopped  down  to  F/8  and  give  four  times  the  exposure, 


Il6  PHOTOGRAPHY  IN  COLOURS 

the  two  plates,  if  developed  together,  should  give 
identical  results. 

Intensification  is  best  done  immediately  before,  or 
after  removal  from  the  clearing  bath.  Either  Lu- 
miere’s  Pyro  and  Silver,  or  the  Mercury  and  Sulphate 
formula  gives  excellent  results ; the  latter  is  some- 
what more  intense  but  less  under  control  than  the 
former.  Both  may  be  used  one  after  the  other  if 
great  intensification  is  desired.  Lumiere’s  formula 
(indicated  by  the  letters  E and  G in  his  original 
instructions  for  developing  Autochromes)  is : E,  Pyro 
1 gm.,  Citric  Acid  1 gm.,  distilled  water  30  c.c. ; G, 
Silver  Nitrate  1‘65  gm.,  distilled  water  30  c.c.  (Note, 
add  G to  the  water  before  E,  or  you  may  get  a pre- 
cipitate.) Eor  use  take  5 c.c.  of  each  and  pour  into  55 
c.c.  of  distilled  water.  Pour  over  the  plate  immediately 
the  solution  is  made  (or  the  solution  will  turn  black), 
rock  and  examine  from  time  to  time  until  sufficiently 
intense,  or  until  the  bath  becomes  turbid.  (See  Ap- 
pendix, Table  11.) 

The  mercury  and  sulphite  intensifier  (see  Appendix, 
Table  11)  is  a very  safe  and  reliable  one.  It  may  be 
varied  in  several  ways  after  the  first  bath.  You  may 
use  either  Sulphite  of  Sodium  or  Sulphite  of  Po- 
tassium 5 % solution  in  water,  or  you  may  re-develop 
with  any  developer  you  choose,  except  Pyro  (which 
has  too  much  colour).  Eerrous  oxalate 1 produces  a 
beautiful  blue-black  deposit  which,  by  quite  obscuring 
the  complementary  colours,  will  give  very  vivid  tones. 

1 Ferrous  sulphate,  saturated  solution  1 part. 

Oxalate  of  potassium  do.  do.  4 parts. 

Leave  to  settle,  and  pour  lear  liquid  for  use. 


SINGLE  COLOUR-SCREEN  PLATES  I I 7 


In  this  latter  case  it  is  well  to  soak  the  plate  im- 
mediately afterwards  in  a dilute  solution  of  Citric  Acid, 
to  dissolve  away  any  oxalate  of  lime  which  may  be 
formed  by  the  oxalate  of  Potassium  uniting  with  the 
lime  in  the  tap  water. 

Lastly,  Piper  and  Carnegie’s  Chromium  intensifier 
may  be  recommended.1  The  author  does  not  advise 
a Uranium  intensifier,  as  it  upsets  the  balance  of 
colours  and  results  in  a mess.  After  intensification 
it  is  occasionally  advisable  to  use  a clearing  bath  of 
neutral  Permanganate  1 % solution,  which  will  at 
once  remove  the  yellow  stains  which  creep  over  the 
whites  during  prolonged  intensification.  If  the  colours 
are  not  impure  the  clearing  solution  may  be  omitted, 
but  the  fixing  bath  must  be  used  if  the  plate  has  been 
intensified  by  the  Pyro-silver  method.  This,  according 
to  Lumiere,  is  essential  if  the  plate  has  been  intensified 
at  all,  otherwise  it  may  be  omitted. 

§ 63 — 16.  Reduction. — This  is  often  needed  when 
the  shadows  are  too  dark  and  obstruct  the  colours,  or 
the  high  lights  are  obscured ; also  in  portraiture  when 
the  hands  and  face  are  too  brown.  It  may  be  effected 
in  several  ways.  (See  Appendix,  Table  12.) 

1st.  By  immersion  in  the  Acid  Permanganate  or  the 
Acid  Bichromate  bath,  diluted  1 to  10  parts  of  water.2 
The  image  must  be  constantly  examined  and  plate 
flooded  with  water  the  moment  the  reduction  is  suffi- 
cient. 

1 This  is  sold  in  tabloid  form  by  Burroughs  Wellcome  & Co., 
Holborn  Viaduct,  and  all  Photographic  Chemists. 

2 The  undiluted  bath  must  not  be  used  for  this  purpose,  or  it 
will  take  the  whole  image  away. 


1 18 


PHOTOGRAPHY  IN  COLOURS 


2nd.  By  using  the  Ammonia  Persulphate  reducer. 

3rd.  Farmer’s  solution. — This  solution,  so  valuable 
with  ordinary  negatives,  is  apt  seriously  to  reduce  the 
brilliancy  of  the  colours  owing  to  the  Hypo  dissolving 
the  bromide  of  silver  unacted  on  by  the  2nd  developer 
(unless  the  redevelopment  has  been  very  thorough). 
If  this  precaution  is  attended  to  the  result  is  most 
satisfactory. 

The  mischief  can  usually  be  remedied  by  reintensifi- 
cation. Whatever  reducer  is  used,  the  action  must  be 
very  carefully  watched,  and  it  is  well  to  begin  with  a 
weak  one. 

The  positive  is  then  washed  and  allowed  to  dry. 

§ 64 — 17.  Drying  the  Positive. — This  may  be  done 
by  fixing  it  on  a whirler  or  in  front  of  an  electric-driven 
fan,  which  dries  it  very  rapidly.  The  plate  may,  how- 
ever, be  dried  just  as  well  but  more  slowly  by  shaking 
it  a few  times,  drying  the  back  and  placing  it  verti- 
cally against  a support.  If  after  a few  minutes 
any  large  drops  have  collected  on  the 
surface  they  must  be  shaken  off,  since, 
owing  to  the  restricted  washing  imperative  with  these 
plates,  the  surface  water  still  contains  traces  of  impurities, 
which  on  evaporation  will  give  rise  to  faint  marks. 

One  must  never  attempt  to  hasten 
drying  by  holding  the  negative  to  the 
fire.  If  there  are  any  drops  of  water  on  the  film,  these 
will  become  heated  and  melt  the  film  beneath,  leaving 
a nearly  transparent  spot  or  a blurring  of  the  image. 
In  any  case  the  experiment  is  a risky  one.  Drying 
with  blotting-paper  is  also  dangerous.  Soaking  in 
alcohol  to  hasten  evaporation  may  cause  the  colours 


SINGLE  COLOUR-SCREEN  PLATES  I 1 9 


to  run.  Hence,  natural  evaporation  is  the  only  safe 
procedure.  At  a temperature  of  65°  an  Autochrome 
plate  should  be  dry  in  30  to  40  minutes.  Omnicolore 
and  Thames  plates  will  take  considerably  longer, 
owing  to  the  thickness  of  the  films. 

§ 65 — 18.  Varnishing-. — As  I have  stated,  varnishing 
is  not  necessary,  but  it  brightens  the  image  and  makes 
the  colours  more  vivid,  and  prevents  the  heat  of  the 
projection  lantern  from  injuring  the  film.  It  requires 
an  experienced  hand  to  do  it  properly  without  leaving 
lines  and  ridges  of  varnish,  which,  unfortunately, 
are  not  entirely  transparent.  If  they  do  occur,  the 
best  plan  is  to  lay  the  plate  in  a bath  of  benzine,  when 
the  ridges  will  dissolve  and  disappear.  As  a rule  the 
difficulty  of  varnishing  arises  from  the  varnish  being 
too  thick.  In  this  case  thin  it  down  with  some  benzole, 
McIntosh  recommends  pouring  the  varnish  over  the 
positive  while  on  a whirler.  He.  commences  the  whirl- 
ing before  it  has  had  time  to  set.  The  author  believes 
that  an  alum  and  citric,  or  chrome  alum  bath  will 
clear  and  harden  the  film  quite  effectively,  and  the 
heat  of  the  lantern  will  not  crack  the  film  more  than 
a varnished  non-hardened  one.  Spotting  out  should 
be  done  by  means  of  transparent  colours,  otherwise 
whatever  colour  you  use  will  appear  black  when  you 
view  the  positive  as  a transparency.  You  may  mix 
the  paint  with  a drop  of  gum  or  oxgall,  to  allow  of 
its  biting  the  glass.  The  finest  sable-hair  brush  pro- 
curable should  be  used. 

§ 66 — 19.  Covering  the  Positive. — The  film  should 
finally  be  protected  by  a plate  of  glass,  and  the  two  plates 
bound  together  with  plaster  strapping.  The  ordinary 


120 


PHOTOGRAPHY  IN  COLOURS 


paper  binding  strips  are  not  to  be  recommended. 
They  take  a lot  of  time  to  put  on.  They  do  not 
keep  the  dust  or  damp  out,  which  creeps  in  at  the 
corners,  and  they  are  in  many  other  respects  un- 
satisfactory. By  far  the  best  method  is  as  follows  : — 
Procure  a long  strip  of  black  or  yellow  rubber  self- 
sticking  surgical  plaster,  which  can  be  obtained  at  any 
chemist’s  shop.  It  is  sold  on  flat  reels,  each  holding 
about  ten  yards,  and  about  fin.  (1  cm.)  wide.  In  order 
to  bind,  pull  out  enough  plaster  to  go  round  all  four 
sides  of  the  plate.  Lay  it  flat  on  the  table,  sticky  side 
uppermost.  Take  the  positive  covered  with  the  pro- 
tecting glass  in  both  hands.  Place  one  side  carefully 
midway  between  the  edges  of  the  plaster.  Then  lift  up 
the  reel,  keeping  the  plaster  on  the  stretch,  and  rotate 
the  positive  with  its  cover-glass  over  each  edge  in 
turn,  keeping  the  plaster  taut  all  the  time,  until  you 
arrive  at  the  point  where  you  began.  Cut  the  tape 
close  to  the  glass,  and  then  press  the  two  sides  of  the 
tape  against  the  sides  of  the  glass.  In  this  way 
two  plates  become  hermetically  sealed,  and  neither 
dust  nor  damp  can  get  in.  The  corners  can  be  pressed 
down  at  once,  and  do  not  need  to  be  trimmed  in  any 
way.  The  whole  process  takes  about  twenty  seconds 
to  perform,  and  the  result  is  admirable. 

Of  course,  the  same  method  will  do  equally  well  in 
the  case  of  lantern  slides.  Seabury  and  Johnson  make 
a good  plaster,  which  is  known  as  Mead’s  plaster.  A 
German  firm  make  an  excellent  black  rubber  plaster 
known  as  Gummi-Pflaster,  which  is  even  better  than 
Mead’s.  1 

A mask  of  black,  brown,  or  olive-green  paper  with 


SINGLE  COLOUR-SCREEN  PLATES  121 


an  opening  of  any  desired  shape  may  often  be  found 
useful  to  hide  any  defects  or  superfluities  in  the  picture. 
It  is  cut  to  fit  the  size  of  plate,  and  placed  over  the 
film  before  covering  with  the  protecting  glass.  When 
finished,  the  picture  may  be  rendered  much  more 
effective  by  a suitable  frame  of  metal,  gilt,  or  dark 
wood,  or  by  one  of  the  many  devices  now  on  the 
market  for  screening  the  picture  from  outside  light,  or 
examining  it  by  reflection  in  a mirror. 

§ 67 — 20.  Final  Improvement  of  the  Tones  of 
the  Image. — The  protecting  cover-glass  affords  a 
means  of  correcting  the  general  tone  of  the  picture. 
Thus,  if  there  is  an  excess  of  blue  in  the  foliage,  it  may 
be  corrected  by  using  the  palest  tinted  yellow  glass.  If 
the  red  tone  is  redundant,  a very  pale  shade  of  green 
(or  of  cobalt-blue  glass  if  orange),  may  be  used  for  the 
cover.  The  former  will  strengthen  the  yellows,  the 
latter  the  blue  of  the  sky.  Another  good  plan  is  to  coat 
a separate  sheet  of  glass  with  a thin  layer  of  gelatine, 
which  you  can  stain  any  colour  you  please.  Thus,  for 
a yellow  tint  use  a pale-tinted  bath  of  Porrier’s  Orange 
II.  For  a pink  tint,  use  Carmine.  For  a blue,  Victoria 
blue  or  Turkey  blue.  For  a greenish-blue,  Methylene 
blue.  The  aim  is  always  to  select  the  complementary 
colour  to  the  one  you  wish  to  counteract  or  modify. 
Often  charming  and  totally  unexpected  results  can  be 
achieved  in  this  way.  Thus,  if  the  transparency  is  too 
blue,  a little  yellow  in  the  coating  will  produce  a 
greenish  tint,  and  an  orange  red  will  help  a sunset, 
and  so  on. 

Dyeing  the  film  of  the  transparency  itself  has  been 
recommended  by  Von  Hubl,  Konig,  and  others,  but  it 


122 


PHOTOGRAPHY  IN  COLOURS 


is  open  to  great  objections.  Thus,  you  may  easily 
over  stain  the  film,  or  the  stain  may  penetrate  into 
the  starch  grains  with  disastrous  results,  or  the  effect 
may  not  be  pleasing,  and  cannot  be  altered. 

§ 68 — 21.  Binding  up  the  Colour=Screen  and 
Transparency  of  Thames  or  Paget  Separate 
Plates. — This  is  easily  learnt,  but  requires  a little  man- 
oeuvring. If  you  place  two  screens  in  contact,  coloured 
side  to  coloured  side,  and  hold  them  up  to  the  light, 
you  will  notice  the  discs  of  the  screens  (if  you  are 
using  Thames  screens)  will  form  a coloured  moire 
pattern  which,  as  you  rotate  the  one  or  the  other 
grows  rapidly  larger  and  larger  until  the  pattern  loses 
all  shape  and  vanishes.  If  you  continue  the  rotation, 
the  pattern  will  again  become  rapidly  smaller.  Now 
substitute  the  transparency  for  one  of  the  screens, 
placing  the  film  against  the  coloured  side  of  the  screen 
as  before,  and  move  it  until  the  pattern  just  vanishes, 
Fix  the  two  with  a spring  letter  clip  and  hold  them  to 
the  light.  If  the  colours  are  not  quite  right  you  must 
shift  the  positive  ever  so  little  one  way  or  the  other 
until,  when  holding  the  pair  squarely  in  front,  the 
colours  are  correct.  Now  fix  three  out  of  the  four 
sides  with  clips  and  proceed  to  bind  the  remaining  side 
with  black  adhesive  binder.  Then  place  a fourth  clip 
over  the  binding  and  carefully  remove  the  opposite 
clip.  In  this  way  registration  may  be  secured  without 
any  shifting. 

The  effect  of  the  picture  is  greatly  enhanced  by 
having  it  mounted  in  a wooden  frame  of  plain  black 
moulding,  about  2J  to  3J  ins.  in  diameter,  and  curved 
so  as  to  throw  the  picture  into  a recess,  as  it  were.  It 


SINGLE  COLOUR-SCREEN  PLATES  1 23 

makes  more  difference  to  the  appearance  of  the  picture, 
than  even  a heavy  broad  gilt  frame  does  to  an  oil  painting. 

§ 69 — 22.  Defects  in  Colour  = Plate  Positives — 
their  Causes  and  Remedies. — (1)  Underexposure. 

— This  may  be  partly  guessed  by  the  length  of  time  which 
has  elapsed  before  the  image  begins  to  appear.  After 
development,  the  image  appears  incomplete,  hard  and 
without  details,  buried  as  it  were.  After  reversal  the 
positive  appears  dark,  dull,  and  heavy,  and  there  is 
absence  of  all  detail  in  the  shadows.  The  remedy  is 
to  reduce  with  weak  Acid  Permanganate,  Parmer’s 
Solution,  or  Ammonium  Persulphate. 

(2)  Over = exposure. — The  image  flashes  up  instead 
of  slowly  becoming  visible  here  and  there  and  gathering 
strength  gradually  over  the  plate.  When  the  image  is 
examined  by  reflected  light  after  it  is  put  in  the 
reversing  bath,  it  appears  loaded  with  details  and  very 
dark.  After  reversal,  owing  to  so  much  of  the  silver 
being  dissolved  away,  the  image  appears  weak,  then 
transparent,  and  much  of  the  detail  eaten  away. 
Remedy. — The  moment  you  find  the  image  coming 
up  too  quickly  in  the  first  developer,  or  the  picture 
appearing  at  once,  flood  the  plate  with  water,  and 
prepare  a freshly  diluted  developer  with  5 to  20  drops 
to  the  ounce  of  a 10  % solution  of  Bromide  of 
Potassium,  and  renew  development,  watching  the 
image  carefully.  Stop  as  soon  as  detail  appears  in 
the  shadows. 

(3)  Yellow  Stains  and  Dichroic  Fog. — If  the 

development  be  prolonged  unduly,  or  too  strong  a 
developer  used,  the  whites  may  become  stained  yellow. 
Remedy. — Bathe  the  plate  freely  in  1/1000  Neutral 


124 


PHOTOGRAPHY  IN  COLOURS 


(i.e.  non-acid)  Permanganate  of  Potash,  followed  by 
half  a minute  in  a fresh  Hypo  bath  containing  Bisulphite 
of  Soda. 

(4)  Brown  Stain. — Cause. — (a)  Immersing  plate  in 
a bath  of  neutral  Permanganate  of  Potash  before  all 
the  developer  has  been  washed  out.  This  is  especially 
liable  if  Pyro  has  been  used  as  the  developer.  It  can 
never  be  entirely  got  rid  of.  (b)  The  picture  has  been 
intensified  by  the  silver  method  before  the  developer 
has  been  washed  out,  or  the  positive  has  been  left  too 
long  in  the  Pyro-silver  bath.  Remedy — Wash  thoroughly 
and  immerse  in  10%  sulphite  of  soda.  If  that  fails, 
place  in  the  reversal  bath  diluted  1 to  10  with  water 
and  then  into  Alum  and  Citric  Acid  bath. 

(5)  Black  Spots. — Cause. — Insufficient  action  of  per- 
manganate reversing  solution,  by  which  small  collec- 
tions of  reduced  blackened  silver  particles  remain 
undissolved.  To  avoid. — Examine  the  transparency 
carefully  before  redeveloping,  and  if  you  see  any  black 
spots  put  the  plate  back  in  the  permanganate  bath  for 
a moment.  Remedy. — Sponge  the  surface  of  the  film 
over,  both  in  the  first  bath  and  permanganate  bath, 
with  a soft  cotton-wool  pad  dipped  in  water.  Pick 
the  spots  off  lightly  with  a needle,  or,  better  still, 
a fine-pointed  penknife.  Lumiere  advises  them  to 
be  dissolved  out  by  means  of  a fine  camel-hair 
pencil  dipped  in  strong  acid  permanganate  solution  or 
in  a mixture  made  up  of  Potassium  Iodide  3 gms., 
Iodine  1 gm.,  water  50  c.c.  This  is  too  delicate  an 
operation  for  most  people,  as  the  liquid  is  likely  to 
spread  a little,  leaving  a washed-out  spot  much  larger 
than  the  black  speck.  In  any  case  the  camel-hair 


SINGLE  COLOUR-SCREEN  PLATES  I 25 


pencil  should  be  made  of  as  few  hairs  and  as  fine  as 
possible.  The  former  method  is  quite  easy,  safe,  and 
effective,  therefore  why  run  any  risks  ? 

(6)  White  Spots  may  be  due  to  minute  thickened 
specks  of  unsensitised  emulsion.  Remedy. — Scrape  off 
with  the  point  of  a penknife.  If  due  to  bubbles,  fill  in 
with  colour  after  varnishing. 

(7)  Green  Spots. — Cause. — Abrasions  in  the  film  and 
access  of  water,  which  dissolves  out  the  green  colouring 
matter  and  spreads  in  an  irregular  circle  round  the 
crack.  Also  washing  the  plate  for  too  long  a time,  by 
which  the  water  has  soaked  through  to  the  green  stain 
and  caused  it  to  run.  Remedy. — Cut  the  green  spot 
clean  out  and  then  paint  over  space  with  a drop  of 
solution  of  gelatine  and  let  it  dry;  or  put  a smear  of 
gum  on  the  area  removed,  and  fit  in  a bit  of  an  old 
film  of  the  right  colour;  or,  lastly,  varnish  the  plate 
after  the  patches  are  cut  or  scraped  out.  Then  fill  in 
the  desired  tint  with  any  transparent  colour.  ( N.B . — 
Chinese  white  is  opaque  to  light  and  appears  black  in 
the  transparency.)  To  prevent  green  spots  use  alum 
bath  or  1/1000  pure  Formaldehyde  (or  1/400  Shering’s 
Formalin,  which  equals  40  % of  pure  Formaldehyde). 
Either  of  these  baths  may  be  used  before  or  imme- 
diately after  first  development.  Also  shorten  the 
washing  processes  as  much  as  possible,  see  that  the 
water  is  not  warm,  and  above  all  never  put  off  intensifi- 
cation or  reduction  until  after  the  positive  is  dry,  if  you  do 
you  are  sure  to  get  green  spots  (see  Appendix  16). 

(8)  Red  Spots  frequently  occur  in  Omnicolore  plates. 
Remedy. — Pick  them  off  with  a fine-pointed  penknife 
or  needle. 


126 


PHOTOGRAPHY  IN  COLOURS 


(9)  General  Violet  Tone  — Causes. — (i)  Access  of 
traces  of  white  light  to  the  plate  which  have  not  passed 
through  the  lens  filter,  or  through  the  lens  itself,  e.g. 
a pinhole  in  the  bellows,  or  chink  in  the  slide,  or  space 
between  the  slide  and  back  of  camera,  or  minute  traces 
of  white  light  reaching  the  plate  before  or  after  exposure, 
or  in  the  dark  room. 

(ii)  The  yellow  filter  is  too  small,  and  allows  light 
to  creep  in  round  the  edges  when  placed  in  front  of 
the  lens. 

(iii)  The  yellow  filter  may  have  been  forgotten. 

(10)  General  Blue  Tone. — (i)  Under-exposure ; (ii) 
Under-development.  This  latter  may  be  used  locally 
to  give  a blue  sky  and  aerial  misty  effect. 

(11)  Veiled  Fog. — (i)  May  be  due  to  any  of  the 
causes  of  general  violet  tone ; or  to  (ii)  too  much  light 
in  the  dark  room,  or  plate  exposed  too  much  to  the 
light.  If  the  light  reaches  the  film  side  first,  it  pro- 
duces general  grey  fog  in  the  first  developer.  If  it 
reaches  the  film  through  the  starch  grains,  a fog,  the 
colour  of  the  light  which  reached  the  plate,  will  appear. 
Thus,  if  light  from  a red  lamp  reached  the  film  through 
the  starch  grains  screen,  only  the  red  rays  would  pass 
through  and  the  fog  would  be  red.  In  the  same  way 
a green  light  would  cause  green  fog.  Remedy. — These 
all  disappear  in  the  Permanganate  Bath,  but  it  has  a 
tendency  to  thin  the  plate  down  and  to  degrade  the 
colours.  A good  plan  is  to  bathe  plate  for  1-2  minutes 
in  the  following  solution — 

Bichromate  of  Potassium  . 1-2  gms.  (12-20  grs.) 

Hydrochloric  Acid  ....  0*35  c.c.  (6  minims) 

Water 100  c.c.  (3|  ozs.) 


SINGLE  COLOUR-SCREEN  PLATES  12 7 


If  the  positive  shows  the  colours  too  weak,  intensify 
by  one  or  other  of  the  methods  given  in  the  Appendix. 

(12)  If  the  Brightness  of  the  Colours  disappears 
the  moment  the  Plate  is  put  in  the  Fixing  Bath. — 

Cause. — The  second  development  was  either  too  short, 
or  else  the  developer  was  too  weak  or  too  cold  (below 
55°),  the  result  being  that  some  of  the  Bromide  of 
Silver  which  ought  to  have  been  reduced  by  the  second 
developer  has  not  been  acted  on,  and  becomes  dissolved 
away  by  the  Hypo,  leaving  a flat,  weak  image.  It  is 
well,  therefore,  if  you  intend  to  use  a Hypo  bath,  to 
leave  the  plate  in  the  second  developer  at  least  three 
or  four  minutes. 

(13)  Frilling  or  Blisters.— Cause. — (i)  The  use  of 

water  at  too  high  a temperature  (above  65°  F.) ; (ii) 
Careless  handling;  (iii)  Differences  in  temperature  of 
baths.  The  water  may  be  cool  enough,  but  the  dark 
room  much  too  hot.  Remedy. — Chrome  alum,  or 

Formalin  solution  after  first  development.  Also  ensure 
that  the  baths  are  all  about  the  same  temperature. 
Autochrome  plates  are  much  better  coated  than  when 
they  were  first  issued.  They  rarely  frill  or  blister  now, 
unless  the  water  or  room  is  above  68°  F.,  and  green 
spots  are  seldom  met  with. 

(14)  There  may  be  a General  Reddish  Tint. — 

Causes. — (i)  Over-exposure;  (ii)  Too  prolonged  wash- 
ing (should  never  exceed  five  minutes) ; (iii)  The  red 
light  getting  access  to  the  film  through  the  starch 
grains  or  grating.  Remedy. — Bind  up  with  a pale 

greenish-blue  cover-glass. 

(15)  If  the  Film  is  Scratched  or  Broken. — 

Probable  cause. — Friction  of  springs  or  card  against  the 


128  PHOTOGRAPHY  IN  COLOURS 


film.  Preventative. — Do  not  put  the  plates  into  the 
slide  until  absolutely  necessary.  Glue  a raised  border 
round  the  card,  or  place  a piece  of  brown  or  dark 
coloured  tissue  paper  between  the  film  and  the  card. 

(16)  If  the  Background  of  a Portrait  or  Group 
is  of  a Dirty  Brown  or  Grey  Colour  (or  the 
Picture  appears  clogged  up  or  opaque). — Cause. — 
Under-exposure  or  over-development.  Development 
has  been  forced  with  too  strong  a developer,  or  pro- 
longed to  bring  this  detail  out,  resulting  in  a dirty, 
heavy,  impure  drab.  Remedy. — Take  the  portrait  over 
again  with  more  exposure  and  weaker  developer. 

(17)  If  the  Face  of  the  Portrait  appears  Thin 
and  Eaten  away.  — Cause.  — Over-development 
brought  about  by  trying  to  bring  out  details  in  a dark 
background.  Remedy. — Nothing  can  be  done  to  improve 
the  positive.  Expose  another  plate  with  a lighter  back- 
ground or  one  better  illuminated. 

(18)  The  Picture  looks  Thin  and  suffers  from 
Want  of  Detail,  especially  in  the  High  Lights. 
— Cause. — (i)  Over-exposure;  (ii)  Over-development 
in  first  bath.  Remedy. — Take  the  picture  over  again, 
with  less  exposure. 

(19)  The  Picture  looks  Dull  and  Opaque. — Cause. 
— (i)  Under-exposure ; (ii)  Under- development  in  first 
bath.  Remedy. — First  reduce  with  Acid  Permanganate, 
or  the  Persulphate,  or  Farmer’s  reducing  bath  (see  for- 
mulae in  the  Appendix),  then  intensify  with  Lumiere’s 
Silver  and  Pyro  bath  (F.  and  G.),  and  repeat  two  or  three 
times  until  the  image  is  sufficiently  dense.  Bleaching, 
followed  by  Bisulphite  of  Soda  or  Quinomet,  is  very 
effective,  but  it  is  somewhat  risky,  as  it  is  liable  to 


SINGLE  COLOUR-SCREEN  PLATES  I 29 


alter  the  tone  of  the  colours  and  the  contrasts.  For 
the  same  reason  Farmer’s  reducer  might  upset  the 
balance  of  colours  by  forming  a Silver  Ferricyanide, 
which  is  somewhat  irregular  in  its  action.  Lumiere 
wisely  recommends  dilute  Acid  Permanganate  for 
Autochrome  plates,  but  Farmer’s  solution  is  better  for 
thick  films  (e.g.  Omnicolore,  Thames,  and  Dufay). 

(20)  The  Colour  has  nearly  all  disappeared  in 
Places,  and  the  Positive  resembles  an  Ordinary 
Positive. — Cause. — (i)  The  film  has  contracted  through 
the  heat  of  an  illuminating  lamp,  and  slightly  shifted 
the  register  of  the  picture  on  the  starch  grains.  The 
plate  has  been  dipped  in  an  acid  bath  which  has 
decolorised  the  starch  grains.  Too  strong  Citric  Acid 
or  too  long  immersion  in  it  will  take  out  the  colour. 
Remedy. — (ii)  None.  Take  the  photograph  over  again. 

(21)  The  Positive  after  Varnishing  shows  a 
Number  of  Red=orange  Spots. — Cause. — Action  of 
varnish  on  traces  of  developer  left  on  the  positive  when 
the  film,  though  apparently  dry,  still  contains  moisture. 
Remedy. — Dissolve  off  the  varnish  with  benzole  and 
immerse  plate  in  1 : 1000  neutral  Permanganate  of 
Potash.  To  avoid. — Soak  the  plate  in  the  above  so- 
lution before  applying  the  varnish. 

§ 70 — 23.  Copying  Colour  Plates,  i.e.  of  colour 
plates  in  which  the  sensitive  emulsion  film  is  attached 
to  the  colour-screens. 

The  reproduction  of  these  plates  is  carried  out  in 
almost  exactly  the  same  way  as  lantern  slides  from  a 
negative.  There  are  two  ways  of  doing  it.  1st.  The 
colour  transparency  may  be  placed  in  front  of  a 
camera  and  copied  through  a lens.  This  allows  of 

K 


130  PHOTOGRAPHY  IN  COLOURS 


the  size  of  the  copy  being  varied.  If  you  have  a dark 
room  and  a window  facing  the  daylight  which  can  be 
blocked  out  by  a shutter,  so  much  the  better.  The 
author  has  a large  square  hole  cut  out  of  the  shutter, 
into  which  he  can  fit  plate  carriers  of  various  sizes. 
The  transparency  is  fitted  into  a carrier  suitable  to 
the  plate,  and  the  latter  is  kept  in  position  by  two 
buttons.  On  the  daylight  side  of  the  transparency  he 
has  a large  board  inclined  at  45°  and  covered  with  a 
sheet  of  white  paper.  This  reflects  the  light  of  the 
sky  through  the  transparency.  On  a table  facing 
the  shutter  is  a long  extension  camera  with  an  anastig- 
mat  of  large  aperture.  To  copy  the  same  size,  the  lens 
is  placed  midway  between  the  transparency  and  the 
plate,  the  two  being  separated  by  four  times  the  focal 
length  of  the  lens.  A colour-filter  is  placed  imme- 
diately in  front  or  behind  the  lens,  or  between  the 
combination  if  made  of  gelatine.  The  transparency  to 
be  copied  should  have  the  film  side  facing  the  ground 
glass,  otherwise  the  picture  will  be  reversed ; and  of 
course  the  copy  must  be  placed  with  the  glass  side 
facing  the  light.  The  after-treatment  of  the  plate  is, 
in  all  respects,  similar  to  that  of  the  original.  The 
other  way  is — 

By  Contact — This  requires  a very  much  shorter 
exposure  than  the  previous  method,  but  it  is  open  to 
the  objection  that  the  film  of  the  copy  cannot  be 
brought  in  contact  with  that  of  the  original,  but 
requires  to  be  turned  round  so  that  the  light  traverses 
the  colour- screen  of  the  copy  before  it  reaches  the 
film.  This  may  be  obviated  by  using  a small  but 
bright  source  of  illumination.  Lumiere  recommends 


SINGLE  COLOUR-SCREEN  PLATES  1 3 I 


a box  (ABOD)  (Fig.  19).  The  transparency  (0)  and 
copy-plate  (P)  are  placed  in  the  frame  (HI).  The  film 
of  the  transparency  is  placed  in  contact  with  the  glass 
side  of  the  copy-plate.  The  source  of  illumination  is 
a piece  of  magnesium  ribbon  (M)  2-5  mm.  in  width  and 
10  cm.  to  20  cm.  in  length,  according  to  the  density 
of  the  transparency.  This  is  folded  double  and  pushed 
inside  an  iron  wire  spiral  (S)  (made  by  winding  the  wire 
round  a penholder  and  then  stretching  it  until  each 
spiral  is  separated  by  a centimetre  from  the  next) 
(Fig.  19).  The  end  of  the  spiral  is  fixed  by  a screw 


D A 


Fig.  19. — Autochrome  Copying  Camera. 

into  the  support  (G).  According  to  Lumiere  the  object 
of  the  magnesium  in  the  principal  axis  of  the  camera  is 
to  prevent  parallax  owing  to  the  separation  of  the  film 
of  the  copy  from  the  transparency  by  the  thickness  of 
the  glass  support.  It  is  necessary  that  no  other  light 
should  enter  the  camera,  except  that  of  the  magnesium. 
E is  a colour-filter,  and  V a movable  slide. 

In  the  case  of  a Paget  plate  in  which  the  screen 
and  film  are  on  separate  glasses,  the  copy  is  made  by 
contact  with  the  original  negative  taken  through  the 
screen.  This  gives  a positive  which  may  be  bound  up 


1 32 


PHOTOGRAPHY  IN  COLOURS 


with  a new  screen  to  give  the  colour,  or  used  as  a 
monochrome  transparency. 

In  any  case,  copying  a colour-plate  from  a colour- 
plate  is  not  altogether  satisfactory,  because  a third  of 
all  the  light  only  goes  through  eachB  coloured  line  or 
grain  in  the  transparency  to  be  copied,  and  of  this  only 
a third  of  the  light,  or  J of  the  initial  light,  gets  through 
the  second  screen,  since  each  coloured  bundle  of  rays 
is  stopped  by  two  out  of  the  three  colours  of  the  screen. 
Moreover,  in  the  case  of  an  Autochrome,  whenever  a 
coloured  ray  meets  a clump  of  the  complementary  colour, 
the  light  is  all  absorbed,  and  the  result  is  a black  dot  in 
the  copy  at  this  spot  after  reversal. 

M.  Gimpel  first  pointed  out  that  whenever  an  Auto- 
chrome made  in  daylight  is  copied  under  the  same 
conditions  with  the  same  light  filter,  the  reproduction 
always  possesses  a predominant  tint,  which  is  usually 
yellow.  This  he  overcomes  by  the  use  of  a pale  violet 
filter.  But  the  difficulty  may  be  overcome  in  another 
way.  Instead  of  reversing  and  making  a positive  copy, 
the  original  negative  may  be  fixed  in  hypo  without 
reversal.  In  this  case  the  inevitable  predominant  tint 
is  reproduced  in  the  copy  in  the  complementary  colour, 
and  if  made  under  the  same  conditions,  the  two  will 
correct  each  other. 

§ 70a. — Achille  Carrara’s  Method. — The  Auto- 
chrome is  placed  in  a Lumiere  copying  camera  (see 
Fig.  19),  in  contact  with  a panchromatic  plate.  Instead 
of  the  Lumiere  filter  he  uses  a set  of  analysis  filters. 
In  front  of  the  filters  he  puts  a sheet  of  fine  ground 
glass  (a  fixed-out  matt  emulsion  plate  does  well).  The 
luminant  preferred  is  a single  filament  of  a Nernst 


SINGLE  COLOUR-SCREEN  PLATES  1 33 


lamp,  5 inches  in  front  of  the  filter.  He  first  makes  an 
exposure  through  the  red  filter.  An  average  density 
Autochrome  requires  30  secs,  exposure  with  the  red 
filter,  and  90  secs,  with  the  green  or  the  blue  filter. 
The  negatives  are  all  developed  together  with  Rodinal 
solution  1-22  for  five  minutes,  preferably  with  the 
addition  of  a few  drops  of  10  per  cent,  bromide.  Prints 
can  be  made  by  any  three-colour  process  such  as 
Eaydex,  Autotype,  or  Sanger-Shepherd’s  methods. 
Sensitising  is  best  done  by  his  quick-drying  Bichromate 
of  Ammonia  solution  1*5  of  water,  of  which  he  takes 
5 c.c.  and  adds  to  it  35  c.c.  of  alcohol  for  a 2 per  cent, 
bath,  20  c.c.  for  a 4 per  cent,  bath,  and  15  c.c.  for  a 
5 per  cent.  bath.  This  he  brushes  rapidly  over  the 
tissue,  which  he  pins  on  to  a board,  three  times  in 
succession,  and  then  it  is  hung  up  to  dry.  In  twenty 
minutes  the  tissue  is  ready  for  use. 

§ 71 — 24.  Indoor  Colour=  Plate  Portraiture. — 
Owing  to  the  prolonged  exposure  necessary  it  is  ex- 
tremely difficult  to  produce  effective  indoor  portraits  in 
colours.  The  difficulty  may,  however,  be  got  over  by 
using  a flashlight  powder  in  conjunction  with  a filter 
specially  adapted  to  the  light  and  the  plate.  The 
Lumiere  Co.  provide  their  “ Ideal  ” Autochrome  Flash- 
powder,  and  a special  “ Auto  P.O.”  filter. 

The  necessary  installation  is  extremely  simple,  and 
may  be  completed  in  a few  minutes  in  a studio  or 
ordinary  room.  It  consists  of — 

(1)  The  “ Ideal  ” Flash-lamp  (L)  (Fig.  20),  which  by 
means  of  a pneumatic  release  fires  a cap,  which  ignites 
the  charge  of  Flash-powder. 

(2)  A strip  of  white  semi-transparent  fabric  (B)  such 


134  PHOTOGRAPHY  IN  COLOURS 


as  muslin,  placed  about  18  inches  from  the  lamp,  to 
diffuse  the  light. 

(3)  A white  screen  (R),  to  reflect  the  light  on  those 
parts  of  the  sitter  not  directly  lighted.  (A  white  fabric 
stretched  on  a frame  is  suitable.) 


Fig.  20. — Diagram  showing  position  of  Camera,  Sitter,  Flash- 
light, and  Screens. 

Copied  by  permission  from  Messrs.  Lumih'e’s  Pamphlet. 


(4)  The  camera  (C),  to  the  lens  of  which  is  fitted  the 
Auto  P.O.  screen. 

(5)  A background  (F)  completes  the  installation.  M 
indicates  the  position  of  the  sitter. 

§ 72 — 25.  Lantern  Projection  in  Natural  Colours. 
This  can  readily  be  done  with  any  of  the  colour 
positives,  made  by  the  Sanger- Shepherd,  Carbon,  or 
Pinatype  processes.  The  Dufay,  Omnicolore,  and 
Thames  Colour  Plates  lend  themselves  admirably  to 
projection,  and  can  be  used  without  further  alteration, 
or  if  too  large  can  be  reduced  by  copying  to  the  lantern 
size  (3^  inches  square).  Autochrome  positives  are  also 


SINGLE  COLOUR-SCREEN  PLATES  I 35 


excellent,  provided  they  are  correctly  exposed  and 
developed,  but  as  a rule  they  are  too  dense  for  pro- 
jection purposes.  In  this  case  they  should  be  reduced, 
either  by  one  of  the  reversal  solutions  diluted  with  five 
or  six  volumes  of  water  or  by  Farmer’s  method  (for 
which  see  tables).  If,  however,  the  plates  are  fully 
exposed,  or  were  slightly  over-exposed  and  correctly 
developed,  they  should  be  quite  thin  enough.  But 
with  these  plates  lime  or  arc  light  should  always  be 
used.  Autochromes  are  often  considerably  improved 
by  intensification,  even  if  the  colours  seem  bright 
enough  when  viewed  by  transmitted  light,  because  the 
enormous  magnification  reduces  the  brilliancy  of  the 
original  very  considerably. 

In  order  to  prevent  the  film  from  melting  or  cracking 
by  the  heat  of  the  light,  it  is  well  to  soak  the  positive 
in  chrome  alum  or  Yon  Hubl’s  solution.  It  is 
doubtful  whether  varnishing  is  any  protection  at  all. 
In  any  case  it  is  as  well  to  place  one  or  two  sheets  of 
mica  immediately  in  front  of  the  condenser,  if  you 
cannot  use  an  alum  trough.  (See  Appendix  17.) 

§ 73 — 26.  Re = sensitising  Colour=screen  Plates. 
— If  a packet  of  plates  happens  to  be  stale,  or  if  it  is 
desired  to  increase  the  sensitiveness  of  a plate,  it  may 
be  revived  in  the  following  way. 

Make  up  the  following  solution  : — 

Alkaline  solution  of  Pinachrome  1 : 500  ...  2 c.c. 


Absolute  alcohol  (96  per  cent.) 50  ,, 

Distilled  water 200  ,, 


Immerse  the  plates  for  two  minutes  in  the  bath  in 
absolute  darkness,  or  faint  green  light.  Dry  rapidly, 


136  PHOTOGRAPHY  IN  COLOURS 

preferably  on  a whirler,  or  in  front  of  an  electric  fan. 
If  neither  of  these  is  at  hand,  shake  the  plate  rapidly 
to  and  fro  for  five  minutes.  If  this  be  not  done,  the 
alcohol  is  apt  to  soak  through  the  plate  and  dissolve 
the  Methyl  Green,  or  else  the  plate  may  become 
fogged.  As  the  result  of  this  treatment  the  reds 
become  rather  too  pronounced,  and  the  greys  become 
a little  too  warm  in  tone,  but  in  every  other  respect 
the  plate  will  resemble  a fresh  one.  By  this  means 
the  plate  will  be  rendered  three  or  four  times  more 
sensitive.  The  red  tone  may  be  got  rid  of  by  using  a 
Dufay  screen  instead  of  an  ordinary  Autochrome  one, 
since  it  is  a purer  yellow  than  the  latter,  and  gives  a 
slightly  colder  tone,  thus  correcting  the  excess  of  red 
which  is  just  what  is  wanted  in  this  case. 

Mr.  T.  H.  Grant  re-sensitises  in  the  following  way. 
He  makes  a drying  cupboard  out  of  a light-tight  box 
of  sufficient  size  to  contain  four  plates  resting  against 
the  sides,  and  to  allow  a small  box  containing  calcium 
chloride,  having  a perforated  lid,  to  be  placed  in  the 
centre,  so  as  to  dry  rapidly  the  plates.  A 7 x 5 
porcelain  dish  to  contain  the  sensitiser,  some  absorbent 
blotting  paper,  and  a whirler  or  electric  fan  complete 
the  outfit. 

Prepare  the  following  solutions.  For  a half  plate 
mix  30  c.c.  of  alcohol  (90  strength)  with  90  c.c.  of 
distilled  water,  thus  reducing  the  strength  to  22  J degrees. 
To  80  parts  of  this  add  10  parts  of  ammonia  solution 
and  10  parts  of  the  dye  solution  as  supplied  by  Mr. 
Charles  Simmen  (to  be  obtained  from  Mr.  Grant,  of 
the  Lumiere  Co.,  89,  Great  Bussell  Street,  W.C.).  The 
plates  are  now  immersed  in  the  bath  for  four  minutes 


SINGLE  COLOUR-SCREEN  PLATES  I 37 


in  complete  darkness,  the  time  being  ascertained  by  an 
alarm  clock,  or  a watch,  observed  by  a screened- off 
dark-room  lamp.  The  dish  should  be  rocked,  and  the 
plates  placed  on  end  in  total  darkness  to  drain.  They 
are  then  whirled  or  fanned,  and  afterwards  placed  in 
the  drying-box  with  the  calcium  chloride  until  the 
morning.  To  hasten  matters,  one  plate  can  be  whirled 
while  another  plate  is  soaking  in  the  dish.  Such  plates 
will  retain  their  sensitivity  for  at  least  a month  after 
being  treated.  Mr.  Grant  says  that  with  these  plates 
he  can  get  good  results  with  an  exposure  of  N second 
at  F/5,  or  N second  at  P/4.  (See  Appendix  18.) 

§ 74 — 27.  Preparing:  Lig:ht  Filters  for  Colour 
Photography  (von  Hubl). — For  a normal  Autochrome 
filter  make  the  following  solutions  : — 

Nelson’s  gelatine  1 : 10  . . . 40  c.c. 

Rapid  filter  yellow  1 : 10  . . 12  ,, 

Echt  Rot  1 : 10 14  „ 

For  use  take  7 c.c.  of  the  dyed  gelatine  for  each 
square  decimetre  of  the  glass  to  be  coated. 

To  correct  the  predominant  blueness  of  neutral  tones 
in  the  shadows,  and  especially  snow  scenes  in  sunshine, 
and  in  all  cases  in  which  insufficient  exposure  is 
unavoidable,  or  on  dull,  cloudy  days,  use  a filter  made 
according  to  the  following  formula  : — 

Gelatine  solution  1 : 10  . , . 40  c.c. 

Rapid  filter  yellow  1 : 10  . . 14  „ 

Echt  Rot  1 : 2000 14  ,, 

The  starch  grains  of  the  Autochrome,  or  even  the 
discs  and  squares  of  most  of  the  other  screen-plates, 


138  PHOTOGRAPHY  IN  COLOURS 

are  not  noticed  when  the  lantern  screen  is  more  than 
10  or  12  feet  from  the  spectator. 

A fine  natural- colour  slide  is  a remarkably  realistic 
and  beautiful  object  when  projected,  and  is  admirably 
suited  for  educational  purposes,  and  making  medical 
records.  Butterflies,  flowers,  portraits,  and  views  with 
plenty  of  colour  in  them,  are  charming  subjects  and 
always  command  the  applause  of  the  spectators. 

§ 75.  Stereoscopic  Effect  of  Colour  Pictures. — I 
have  repeatedly  observed  that  colour  slides  show  a 
marked  stereoscopic  effect  when  projected  on  the  sheet, 
and  many  of  my  friends  have  remarked  the  same  thing. 
It  is  needless  to  add  that  it  is  not  a real  stereoscopic 
projection,  although  the  illusion  is  often  very  striking. 

§ 76.  Colour  Screen  Filters  for  Monochromatic 
Light  . — These  may  consist  of  (1)  Pieces  of  coloured 
glass ; (2)  stained  gelatine,  either  separate  or  coated  on 
glass,  and  protected  by  a second  piece  of  glass,  as  used 
for  the  Autochrome  plate ; or  (3)  thin  glass  troughs 
with  parallel  sides,  containing  the  coloured  fluid  and 
hermetically  sealed. 

Most  of  the  following  substances  or  solutions  have 
been  recommended  by  Professor  R.  W.  Wood.  They 
correspond  to  the  chief  colours  of  the  spectrum,  and 
ultra-violet  and  infra-red  rays,  and  will  be  found  to 
afford  very  effective  monochromatic  light  filters  for 
Microphotography  and  laboratory  work  when  using  the 
Mercury  vapour  lamps. 

Ultra-violet  light  (line  316  to  326)  is  produced  by  a 
chemically  deposited  film  of  silver  on  a quartz  lens  or 
plate.  This  filter  lens  was  employed  by  Wood  when 
photographing  the  moon  and  landscapes  by  ultra-violet 


SINGLE  COLOUR-SCREEN  PLATES  1 39 


light.  The  silver  film  should  be  of  such  thickness  that 
a window  in  front  of  a brilliantly  lighted  sky  is  barely 
visible  through  it. 

Ultra-violet  light  (line  365)  is  obtained  by  a very 
dilute  solution  of  methyl  violet  4 R (Berlin  Anilin 
Eabrik)  and  nitroso-dimethyl  aniline. 

Deep  violet  (line  405)  may  be  made  with  Methyl 
violet  and  Chinin  sulphate  in  separate  solutions. 

Blue  violet  (corresponding  to  the  primary  colour). 
Ammoniated  saturated  solution  of  Sulphate  of  Copper ; 
a solution  of  Sulphocyanate  of  Cobalt. 

Blue  (line  436).  Cobalt  glass  (thick)  and  iEsculine 
solution. 

Green.  Solution  of  bichromate  of  potash  and  a 
solution  of  Neodymium  chloride.  The  Bichromate 
transmits  the  Green  and  the  two  yellow  lines.  The 
Neodymium  absorbs  the  Yellow,  leaving  the  Green. 

Green  (another  line  492).  A mixture  of  Guinea  green 
(B  extra)  and  Chinine  sulphate. 

Yellow  (line  579).  A thick  layer  of  Bichromate  of 
Potash ; or  (2)  a solution  of  Eosin  and  Chrysoidin. 

Deep  red  (line  690).  Very  dense  Cobalt  glass  and  a 
layer  1-2  cms.  thick  of  saturated  solution  of  Potassium 
Bichromate.  This  was  used  by  Wood  for  photograph- 
ing infra-red  landscapes.  A clear  blue  sky  is  nearly 
black  through  it,  and  sunlit  foliage  comes  out  nearly 
white. 

Infra-red.  Saturated  solution  of  Iodine  in  Bisulphate 
of  Carbon.  It  is  quite  opaque  to  the  eye  and  all  visible 
rays,  but  freely  transmits  the  infra-red  rays. 

A good  monochromatic  light  can  be  obtained  by 
saturating  an  asbestos  cylinder  with  a strong  solution 


140  PHOTOGRAPHY  IN  COLOURS 

of  Chloride  of  Lithium.  A good  green  light  may  be 
obtained  by  placing  a bead  of  fused  metallic  thallium 
in  a loop  of  platinum  wire  in  the  extreme  outer  edge  of 
the  flame. 

For  most  work,  the  Mercury  Arc  will  be  found  very 
satisfactory.  (For  further  details  see  Appendix  19, 
p.  285.) 


CHAPTER  IX 


THREE-PLATE  AND  TWO-PLATE  COLOUR  PHOTOGRAPHY 

§ 77.  The  Theory  of  Three-colour  Photography. 

— The  process  of  M.  Lippmann  is  of  scientific  interest 
merely ; comparatively  few  workers  have  attained 
much  success  with  it,  and  it  is  only  to  a very  limited 
degree  suitable  for  exhibition  purposes.  Those  pro- 
cesses, however,  which  depend  on  the  “ three-colour  ” 
principle  are  daily  growing  in  favour,  many  of  the 
positives  being  of  great  beauty.  There  are  two  forms 
of  this  process,  the  “ subtractive  ” one  which  is  worked 
by  Sanger-Shepherd  & Co.,  and  the  “ additive  ” method, 
of  which  the  Lumi6re  Autochrome,  the  Omnicolore,  and 
the  Thames  plates  are  excellent  examples. 

The  principle  of  three  colours  being  used  to  reproduce 
all  colours  was  discovered  independently  by  Fredk. 
Ives,1  of  Philadelphia  (the  inventor  of  the  “ Half-tone  ” 
process,  which  has  revolutionised  the  art  of  illustration 
of  books  and  newspapers),  and  Eucos  du  Hauron,  of 
France.  It  is  founded  on  the  Young-Helmholtz  theory 
of  colour  vision,  elaborated  by  Clerk  Maxwell.  As  this 
theory  is  of  fundamental  importance,  a certain  amount 
of  repetition  will  be  pardoned.  Every  colour  in  nature 

1 One  of  my  critics  in  “ Knowledge  ” (Feb.  1911)  denies  this 
statement,  but  further  investigation  of  the  subject  only  confirms 
what  I have  stated,  viz.  that  he  discovered  it  independently. 


142 


PHOTOGRAPHY  IN  COLOURS 


can  be  formed  from  one  or  more  of  the  three  primary 
colours  (or,  more  strictly  speaking,  coloured  lights),  red, 
green,  and  blue -violet.  Thus,  orange-red  light  and 
green  light,  when  combined,  produce  the  sensation  of 
yellow ; green  and  blue,  that  of  greenish-blue,  orange- 
red  and  blue,  purple;  while  brown  may  be  produced 
by  the  admixture  of  much  red,  a little  green,  and  less 
blue.  The  different  shades  of  these  coloured  lights 
may  be  produced  by  varying  the  intensities  of  the 
mixtures. 

A mixture  of  all  three  coloured  lights  in  the  right 
proportion  will  produce  the  sensation  of  white.  On 
the  other  hand,  the  superposition  of  the  same  colours 
in  the  form  of  pigments  will  produce  the  sensation  of 
black,  since  when  combined  each  pigment  absorbs  the 
colour  which  otherwise  would  be  reflected  by  the  other 
pigment ; hence  no  colour  is  reflected  and  the  result  is 
black. 

Ives  devised  a camera  having  two  reflecting  mirrors 
by  which  three  negatives  of  the  same  object  were 
simultaneously  obtained — one  behind  an  orange  glass 
or  filter,  one  behind  a green  glass  or  filter,  and  one 
behind  a blue-violet  glass  or  filter.  In  a later  form  of 
camera  the  stereoscopic  principle  was  employed,  and 
the  three  pairs  of  positives  were  viewed  in  a stereoscope 
which  he  called  a Kromskop. 

§ 78.  Ives’  Kromskop. — By  using  a triple  lantern 
Ives  superimposed  the  three  coloured  images  on  a 
sheet.  Now,  although  true  stereoscopic  pictures  in 
relief  cannot  thus  be  obtained,  since  one  cannot  combine 
stereoscopic  images  on  a screen  as  is  done  in  the 
Kromskop,  an  apparent  plastic  relief  not  observed  in 


THREE-PLATE  COLOUR  PHOTOGRAPHY  1 43 


black  and  white  slides  is  obtained.  I have  heard  this 
remarked  by  many  people.  The  effect  is  even  more 
pronounced  when  the  picture  is  observed  with  one  eye 
only.  Possibly  the  explanation  lies  in  the  fact  that 
the  colour  increases  the  sense  of  reality  in  the  picture 
and  enables  the  mind  to  supply  the  plasticity  which 
experience  tells  us  must  exist  in  the  actual  object. 
This  is  only  carrying  a step  further  the  well-known 
fact  that  if  we  look  with  one  eye  through  a short  tube 
at  an  engraving  or  painting,  it  will  convey  a sense 
of  plasticity  which  is  wanting  when  the  same  picture 
is  regarded  by  both  eyes. 

Fig.  21  shows  the  essential  parts  of  Ives’  Kromskop. 
A,  B,  and  C are  sheets  of  red,  blue,  and  green  glass 


Fig.  21. — Ives’  Kromskop,  showing  how  the  pictures  are 
combined. 

respectively,  on  which  the  three  pairs  of  stereoscopic 
positives  made  for  these  colours  are  placed.  H,  D, 


144 


PHOTOGRAPHY  IN  COLOURS 


and  E are  mirrors  inclined  at  45°.  E is  one  of  a pair 
of  stereoscopic  lenses,  i.e.  of  two  convex  prism  lenses, 
the  prisms  having  their  bases  directed  outwards,  and 
the  lenses  are  each  of  slightly  longer  focus  than  the 
distance  CE. 

The  mirrors  and  glass  plates  are  so  arranged  with 
respect  to  E that  the  distances  AE  + EE,  BD  4-  BE, 
and  CE  are  all  equal,  so  that  the  images  of  each  colour 
enlarged  by  the  lenses  FE  will  exactly  coincide,  and 
give  rise  to  a single  coloured  aerial  image  in  stereoscopic 
relief  at  the  near  point  of  the  observer.  This  coloured 
image  appears  to  stand  out  in  the  most  vivid  relief,  and 
if  the  three  positives  are  equally  illuminated  by  an 
even  light,  by  means  of  a fine  ground  glass  placed  in 
contact  with  the  outer  side  of  the  three  pairs  of  posi- 
tives, and  the  colours  correctly  chosen,  the  result  is 
exceedingly  beautiful. 

A difficulty  arose  in  connection  with  the  two  mirrors 
marked  D and  E which  Ives  overcame  in  a most 
ingenious  manner.  It  was  necessary  that  they  should 
be  unsilvered,  since  they  had  to  transmit  light,  and 
unsilvered  mirrors  give  at  least  two  images — one  from 
the  front  surface,  and  a second,  not  so  bright,  from  the 
back  surface.  The  difficulty  was  overcome  by  using 
for  each  mirror  a coloured  glass  which  absorbed  the 
kind  of  light  which  entered  it  and  would  otherwise 
have  been  reflected.  For  example,  the  mirror  E 
had  to  reflect  red  light  from  the  red  transparency  A ; 
so  to  quench  the  red  light  that  entered  it,  it  was 
made  of  a blue-green  pot-metal  glass  that  absorbed 
red  light,  i.e.  one  in  which  the  colouring  matter  is 
spread  throughout  the  substance  of  the  glass,  this  being 


THREE-PLATE  COLOUR  PHOTOGRAPHY  1 45 


more  effective  for  the  purpose  than  a flashed  one. 
The  only  light  reflected  from  E was  the  red  light  which 
was  reflected  at  the  first  surface.  Similarly  the  mirror 
D was  made  of  chromium  green  glass  which  absorbed 
the  blue  from  the  blue  positive  B.  On  the  other  hand, 
the  glass  I)  was  transparent  to  the  green  light  from  G, 
while  the  glass  E transmitted  the  light  from  both  the 
green  and  the  blue  transparencies. 

§ 79.  Colour  Filters. — In  order  to  make  negatives 
for  reproduction  by  an  additive  method  such  as  Ives’ 
Kromskop  or  by  any  of  the  subtractive  ones  to  be 
described,  it  is  necessary  to  use  colour  filters  in  order 
to  ensure  that  only  the  light  actually  wanted  shall  be 
recorded  on  the  photographic  plate.  The  colour 
filter  is  understood  to  mean  the  coloured  glass  or 
gelatine  film  placed  in  the  path  of  the  rays  to  exert  a 
selective  action  on  the  light  which  falls  on  the 
photographic  plate.  The  colour  screen  is  a 
plate  covered  with  coloured  dots,  grains,  or  lines, 
and  is  placed  immediately  in  front  of  the  panchromatic 
film.  Neither  “pot-metal”  nor  “flashed”  coloured 
glasses  are  suitable  as  colour  filters,  because  they 
absorb  too  much  light  and  the  colours  do  not  admit 
of  adjustment,  nor  can  coloured  glass  of  the  requisite 
quality  always  be  obtained.  Gelatine  films  stained 
with  aniline  dyes  are  now  generally  employed,  and 
these  are  usually  sealed  in  between  two  pieces  of 
glass.  The  great  number  and  variety  of  such  dyes 
admits  of  almost  any  desired  absorption  being  obtained. 
Some  few  workers  prefer  to  use  the  unprotected  film 
only,  and  to  place  it  in  the  diaphragm  slot  of  the  lens 
fixed  on  a Waterhouse  stop.  Such  an  unprotected  film 

L 


146  PHOTOGRAPHY  IN  COLOURS 

is  very  liable  to  damage,  and  it  is  more  usual  to  seal  it 
between  glasses  with  Canada  balsam  and  to  bake  the 
filter  at  a gentle  heat  for  some  days  in  order  to  make 
the  balsam  set. 

The  glass  that  is  used  for  this  purpose  has  to  be 
plane  and  optically  worked,  or  the  definition  of  the  image 
may  be  impaired.  While  it  is  quite  true  that  the  use 
of  such  glass  plates  introduces  a slight  elongation  of 
the  focal  length,  it  is  generally  even  safer  to  employ 
thick  glasses  than  thin  ones,  for  thin  filters  are  very 
liable  to  distortion,  which  renders  them  useless.  Care 
should  always  be  taken  that  the  filters  are  not  held 
too  tightly  in  their  rings. 

The  position  of  the  filter  is  a matter  of  some  import- 


Fig.  22. — Sanger-Shepherd’s  Three-colour  Plate  Camera. 

ance.  Unless  a special  camera  is  used,  it  is  generally 
most  convenient  to  fix  it  on  the  hood  of  the  lens.  In 


THREE-PLATE  COLOUR  PHOTOGRAPHY  1 47 


this  position  the  light  has  some  distance  to  travel  after 
passing  through  it,  and  therefore  it  must  be  made  of 
good  quality  glass.  On  the  other  hand,  the  filters  may 
be  of  the  same  size  as  the  photographic  plates,  and 
fitted  immediately  over  these  as  in  the  camera  depicted 
in  Fig.  22.  Here  the  optical  quality  of  the  glass  is  not 
of  the  same  importance,  but  they  must  be  very  even 
in  colour  and  have  no  bubbles  or  flaws,  as  shadows  of 
these  would  be  reproduced  in  the  image. 

It  is  always  advisable  to  focus  the  image  through 
one  of  the  filters,  owing  to  the  shift  in  the  position  of 
the  image  produced  by  the  filter,  considered  as  a plate 
of  glass  and  quite  apart  from  its  absorption  effects. 
There  are  two  effects  produced  by  a plate  of  glass 
under  these  conditions.  The  most  obvious  is  the 
alteration  in  the  focus.  If  the  plate  is  placed  behind 
the  lens  the  image  of  a distant 
object  is  thrown  back  by  an 

amount  equal  to  ~~  where 

t is  the  thickness  of  the  plate  and  /x 
its  refractive  index.  This  amount 
is  approximately  one-third  the 
thickness  of  the  plate,  since  /x 
is  always  about  1*5.  The  devia- 
tion of  the  light  is  indicated  in 
Fig.  23.  On  the  other  hand,  when 
the  plate  is  in  front  of  the  lens 
there  is  no  shift  of  the  image  Fig.  23. 

or  alteration  of  focus  unless  the 

object  is  near.  The  second  effect  of  a plate  of  glass 
placed  behind  the  lens  is  to  produce  a slight  alteration 


148  PHOTOGRAPHY  IN  COLOURS 

in  the  size  of  the  image,  and  therefore  it  is  im- 
portant to  see  that  all  the  filters  of  a set  are  of  exactly 
the  same  thickness,  as  any  variation  in  this  respect 
will  throw  the  images  slightly  out  of  register ; a serious 
matter  in  large-sized  pictures  and  process  work. 

§ 80.  The  Testing  of  Colour = plate  Filters. — The 
test  which  is  now  usually  applied  to  filters  is  to  photo- 
graph a spectrum  of  white  light  through  them.  Nega- 
tives of  a spectrum  are  made  upon  the  plate  to  be  used 
through  each  of  the  three  filters,  and  from  these  it  can 
be  seen  exactly  what  light  is  recorded.  This  method  is 
also  valuable,  because  it  at  once  shows  if  any  ultra-violet 
light  is  being  passed  by  the  filters.  This  cannot  be 
seen  by  the  eye,  but  the  plate  is  very  sensitive  to  it. 
The  best  filters  now  in  use  show  under  this  test  a slight 
overlap  between  the  red  and  green  filters  in  the  yellow, 
and  between  the  green  and  blue  filters  in  the  blue- 
green.  There  should  be  no  unrecorded  gaps. 

Much  may  be  learnt  from  a simple  visual  examination 
of  the  filters.  If  the  red  and  green  filters  are  super- 
posed and  a bright  light  is  viewed  through  them,  then  a 
person  of  normal  colour  vision  should  see  a dark  yellow 
colour.  The  blue  and  green  filters  superposed  should 
produce  a dark  bluish-green.  Although  the  ultra-violet 
light  cannot  be  seen,  one  can  ascertain  if  the  red  filter 
passes  any  extreme  violet  by  superposing  the  blue  one 
upon  it,  when  only  a very  deep  red  without  any  violet 
tinge  should  be  seen.  Any  red  filter  that  fails  to  pass 
this  test  is  valueless. 

§ 81.  Making  Three-colour  Negatives.  — When 
the  light  filters  have  been  accurately  chosen,  a plate  is 
exposed  behind  each  filter  in  a camera.  It  might  be 


THREE-PLATE  COLOUR  PHOTOGRAPHY  1 49 


thought  possible  to  take  all  three  pictures  at  once  by 
using  a wide  camera  with  three  lenses,  but  this  is  impos- 
sible, because  unless  the  three  negatives  are  taken  from 
exactly  the  same  spot,  the  copies  cannot  be  accurately 
superposed,  since  the  difference  in  the  point  of  view 
would  give  stereoscopic  images  which  cannot  be  made 
to  coincide.  Each  plate  must  receive  such  an  exposure 
that  a white  object  may  be  represented  by  a deposit 
identical  in  position  and  area  in  each  of  the  three 
negatives.  This  forms  the  key  to  successful  printing. 

Sets  of  three-colour  negatives  may  be  made  with  an 
ordinary  camera,  provided  that  some  simple  holder  is 
made  to  hold  the  filters  one  at  a time  in  front  of  the 
lens.  The  operation  consists  simply  in  exposing  three 
panchromatic  plates  behind  the  red,  green,  and  blue 
filters  successively,  and  then  developing  them  in  the 
ordinary  way.  It  is  most  important  that  the  correct 
exposure  be  given  to  each  plate,  so  that  a scale  of  greys 
is  rendered  in  the  same  manner  in  all.  To  judge  of 
this,  trial  exposures  may  be  made  on  a piece  of  crumpled 
white  blotting-paper,  and  the  exposures  altered  until 
the  images  of  it  in  the  three  negatives  are  identical. 

A very  convenient  attachment  to  a camera  for  this 
class  of  work  is  a repeating  back  having  a long  dark 
slide  to  hold  the  three  plates,  or  one  long  plate,  and  a 
frame  holding  the  filters  in  front  of  it.  The  frame  and 
the  dark  slide  move  along  together,  so  that  plate  and 
filter  are  both  changed  by  the  one  movement  (Eig.  24). 

§ 82.  Butler’s  Three=plate  Camera.— Mr.  E.  T. 
Butler  has  designed  a useful  camera  on  the  principle  of 
Ives’  Kromskop.  It  will  be  found  very  useful  for  pro- 
curing the  negatives  for  the  Sanger-Shepherd  method. 


150  PHOTOGRAPHY  IN  COLOURS 

The  camera  is  of  the  box  form  and  fitted  with  grooves 
to  hold  three  double-backs,  two  above  and  one  behind 
(Fig.  24).  The  first  sensitive  plate,  F,  has  a red  filter 
in  contact  with  it,  the  second,  G,  a bine  filter,  both 
made  of  patent  plate,  but  the  third  sensitive  plate,  H, 
has  none  at  all.  In  order  that  the  light,  after  passing 
through  the  lens  should  reach  the  plates  F and  G two 
glass  plate  reflectors  placed  at  45°  are  required.  Since 


Fig.  24. — Diagram  of  path  of  light  in  Butler’s  Three-plate 
Camera. 

both  the  front  and  back  surfaces  of  a glass  plate  reflect 
light,  it  would  give  rise  to  double  images,  were  it  not  in 
some  way  prevented.  Ives  got  over  the  difficulty  by 
employing  thin  wedge-shaped  reflectors  and  covering 
their  backs  with  coloured  varnish.  Butler  has  got  over 
the  double  images  by  employing  two  reflectors  set  at 
an  angle  of  45°  to  the  axis  in  the  following  ingenious 
way.  The  first  reflector  consists  of  bluish-green  glass 
(the  complementary  to  red)  which  absorbs  the  red 


THREE-PLATE  COLOUR  PHOTOGRAPHY  I 5 I 


light  and  transmits  the  blue-violet  and  green  rays  only. 
Thus  the  white  light  after  passing  through  the  lens 
reaches  the  first  plate  and  is  partly  reflected  directly 
upwards,  and  partly  reflected  and  partly  refracted  from 
the  back  surface  of  the  plate.  On  undergoing  a second 
refraction  into  air  the  ray  passes  up  parallel  to  the 
main  pencil.  On  reaching  the  red  sheet  placed  im- 
mediately in  front  of  the  sensitive  plate,  the  green  and 
blue  rays  are  absorbed  while  the  red  rays  pass  through, 
thus  all  the  light  reflected  from  the  back  of  the  plate  is 
absorbed  and  never  reaches  the  sensitive  film  at  all. 

In  the  same  way  the  second  reflector  consists  of 
yellow  glass  the  complementary  to  the  blue,  so  that 
the  rays  reflected  from  the  back  of  this  plate  are 
absorbed  by  the  blue  filter  and  only  the  blue  rays 
reach  the  second  sensitive  film.  Thus,  again,  the  rays 
reflected  from  the  back  of  the  plate  become  absorbed 
and  never  reach  the  film.  The  remainder  of  the  light, 
which  is  green  (since  it  has  lost  its  red  and  blue  con- 
stituents), passes  directly  on  to  the  third  panchromatic 
plate,  and,  of  course,  requires  no  filter  in  front. 

The  order  of  the  three  coloured  filters  must  be  so 
arranged  that  each  will  get  its  proper  share  of  the  light. 
Hence  the  red  plate  which  requires  a long  exposure 
must  have  the  greatest  volume  of  light.  It  is  therefore 
placed  at  F,  so  as  to  receive  the  full  reflected  beam  of 
white  light.  The  green  filter  is  really  in  the  brightest 
position,  since  it  receives  all  the  transmitted  light. 
Since  a red  filter  can  be  made  which  lets  all  the  red 
through,  but  a green  filter  cannot  be  made  to  let  all  the 
green  through,  it  has  been  found  best  to  keep  the  green 
for  the  direct  light.  Of  course,  with  this  camera  the 


!52 


PHOTOGRAPHY  IN  COLOURS 


exposure  is  the  same  for  all  three  negatives  and  cannot 
be  varied  as  is  the  case  with  other  cameras. 

The  negatives  which  receive  reflected  light,  viz.  the 
red  and  blue  ones,  will  be  reversed,  whereas  the  green 
negative  which  receives  direct  rays  will  not  be  so.  This 
may  be  rectified  by  turning  the  green  negative  film-side 
out,  taking  care  to  allow  for  the  change  of  focus  owing 
to  the  thickness  of  the  glass  support. 

§ 83.  Two-plate  Colour  Photography. — The 
difficulties  attendant  on  three-colour  photography,  and 
especially  on  making  all  three  exposures  at  one  time, 
have  led  to  attempts  being  made  with  two  colours. 
Gurtner  has  invented  and  patented  a very  simple  process, 
which,  while  ignoring  the  red  element,  still  enables  one 
to  produce  charming  pictures  of  natural  scenery.  He 
first  takes  a chlorobromide  emulsion  plate,  very  thinly 
coated  (in  other  words  a lantern  plate),  stains — in  the 
dark  in  a bath  of  naphthol  orange,  or  aurantia,  dissolved 
in  water — dries  it,  and  then  places  it  in  contact  with  a 
panchromatic  plate,  film  to  film.  The  two  plates  are 
then  placed  in  the  dark  slide,  taking  care  that  the  glass 
side  of  the  lantern  plate  faces  the  lens.  The  ground 
glass  is  reversed,  as  is  done  when  taking  an  autochrome 
picture  in  order  to  compensate  for  the  thickness  of  the 
glass,  and  an  exposure  made  in  the  ordinary  way. 
The  orange  lantern -plate  absorbs  the  blue  rays,  which 
act  on  the  plate  and  form  the  image,  and  allows  the 
red,  yellow,  and  green  rays  to  pass  through  the  semi- 
transparent film.  These  act  on  the  panchromatic  film, 
and  form  a second  image  by  the  action  of  the  red  and 
orange-green  rays.  The  orange  plate,  which  has  a 
dark  image  when  the  blue  rays  have  acted,  serves  to 


TWO-PLATE  COLOUR  PHOTOGRAPHY  1 53 


print  the  yellow  image,  while  the  panchromatic  plate, 
which  gives  a dark  image  under  the  red-green  rays, 
serves  as  the  negative  for  the  blue  image.  A little 
trouble  is  necessary  to  adjust  the  density  of  the  orange 
stain  so  as  to  give  the  relatively  correct  exposures  for 
the  two  plates.  In  fixing,  the  yellow  stain  dissolves 
out.  A print  is  then  obtained  from  the  panchromatic 
plate,  either  by  making  a positive  and  staining  it  blue, 
or  by  making  a blue  print  on  a ferro-cyanide  paper 
direct.  A positive  is  made  from  the  lantern  plate 
(which  has  now  lost  its  colour)  either  on  a second 
lantern  plate,  or  on  a detachable  celloidin  paper.  These 
copies  are  best  fixed  in  ammonia,  without  being  toned, 
and  stained  lemon  or  orange-yellow.  The  blue  and 
yellow  glass  positives  are  now  dried  and  placed  in 
position,  face  to  face  ; and  bound  together  like  a lantern 
slide  with  binding  adhesive  paper.  If  a paper  print 
is  required,  the  celloidin  print  is  squeezed  down  on  to 
the  blue  paper  print,  after  careful  adjustment. 

The  results  of  this  process  are  often  very  satisfactory, 
and  it  has  the  advantage  of  simplicity,  since  any 
ordinary  camera  will  suffice,  no  filters  or  dyes  are 
needed,  as  is  the  case  with  three-colour  processes ; and 
only  two  prints  need  putting  into  register.  The  ferro- 
cyanide  (blue-printing)  paper  can  be  obtained  in  packets 
from  any  dealer.  It  is  very  useful  to  judge  the  effect 
of  a negative,  as  the  prints  are  fixed  in  ordinary  water, 
and  are  made  in  a minute. 


CHAPTER  X 


THREE-PLATE  PHOTOGRAPHIC  COLOUR  PRINTING 

§ 84.  Colour  Prints. — The  colour  processes  hitherto 
described  only  furnish  single  diapositives,  i.e.  transpar- 
encies in  which  the  picture  is  illuminated  from  behind, 
and  seen  by  one  person  at  a time,  or  projected  on  to  a 
screen.  But  people  naturally  call  for  pictures  which 
can  be  hung  up  on  a wall,  or  placed  in  an  album,  and 
seen  by  reflected  light.  Such  pictures  also  should  be 
capable  of  reproduction.  These  two  problems  are  by  no 
means  so  easy  of  solution  as  would  appear  at  first  sight, 
although  there  are  quite  a number  of  ways  by  which 
they  may  be  accomplished.  Some  of  these  methods 
yield  at  best  only  poor  results,  while  others  require  an 
amount  of  experience  and  care  which  is  possessed  by 
very  few  persons.  The  following  processes,  however, 
are  quite  successful  in  the  hands  of  careful  workers. 

§ 85.  Practical  Details  for  working-  the  Three= 
Plate  Method  with  Butler’s  Camera. — Mr.  Butler 
prefers  Wratten  and  Wainwright’s  Panchromatic  plates 
for  all  three  negatives,  or  Wratten’s  Panchromatic  plate 
for  the  red  exposure,  and  the  Gem  “ Tricol  ” for  the 
green  and  blue.  But  any  ordinary  plate  will  do  for 
the  blue  just  as  well. 

Having  inserted  the  three  plates  into  the  camera 
behind  the  three  filters,  as  described  in  § 81,  proceed 


THREE-PLATE  COLOUR  PRINTING  1 55 


to  judge  the  exposure  by  a Watkin’s  or  Wynne’s 
actinometer.  If  the  paper  changes  until  it  matches 
the  dark  green  sector  in  any  time  up  to  45  seconds  or 
even  a minute,  you  can  expose  as  the  actinometer 
indicates ; but  if  the  time  exceeds  one  minute,  double 
the  exposure.  If  it  exceeds  two  minutes,  multiply  the 
exposure  by  three.  Sunsets  require  from  5 seconds  to 
15  seconds,  with  an  aperture  of  from  F/4  to  F/6. 

Each  colour  is  represented  on  each  negative  in  vary- 
ing densities  of  the  silver  images  in  the  same  way  as 
if  each  plate  were  exposed  separately,  and  of  course 
each  print  must  be  made  in  the  complementary  colours 
to  the  filters  through  which  the  pictures  were  taken. 

Method  of  Procedure. — Three  negatives  are  first 
made  in  the  camera  : 

No.  1 negative  (the  red),  making  the  blue  print. 

No.  2 negative  (the  green)  „ red  ,, 

No.  3 negative  (the  blue)  „ yellow  ,, 

No.  1 negative  is  taken  direct,  and  a print  made  from 
it  by  contact  will,  of  course,  face  the  right  way.  It  is 
placed  in  the  enlarging  apparatus  (glass  side  to  the 
lens),  and  the  exposure  is  made  in  the  usual  way  to 
make  a positive.  With  daylight  the  ordinary  acti- 
nometer tint  must  be  used,  which  may  be  halved  or 
doubled.  Develop  with  Metol-Hydroquinone  for  the 
black  tone  lantern  plate  which  is  to  record  the  enlarged 
positive.  Develop  until  all  details  are  out,  but  stop 
immediately  the  high  lights  begin  to  veil  over.  The 
positive  will  be  thin,  but  quite  strong  enough  if  the 
details  show  up  when  the  positive  is  placed  on  a white 
ground.  Fix  in  Hypo,  wash  well  in  running  water  for 
a quarter  of  an  hour,  then  place  in  an  8 per  cent. 


156  PHOTOGRAPHY  IN  COLOURS 

solution  of  Ferricyanide  of  Potassium  (red  prussiate) 
until  bleached.  Wash  well  for  5 minutes,  then  place 
in  an  8 per  cent,  solution  of  Perchloride  of  Iron  for  one 
minute.  Wash  well  for  30  seconds,  then  place  in  Hypo 
for  one  minute.  Wash  again  in  running  water  for  5 
minutes.  Place  for  a moment  in  a weak  solution  of 
Sulphuric  Acid  (a  few  drops  to  8 ozs.  of  water)  until 
the  yellow  stain  is  removed.  Finally  wash  well  for 
3 minutes  to  get  rid  of  the  acid.  This  forms  the  posi- 
tive from  which  the  blue  picture  is  obtained. 

No.  2 negative  (red  printer)  is  placed  in  the  enlarging 
apparatus,  glass  side  to  the  lens,  and  a copy  made  on 
a black-tone  lantern  plate,  as  was  done  in  the  case  of 
No.  1 negative. 

No.  3 negative  (yellow  printer)  is  placed  glass  side 
to  the  lens  as  before,  and  similarly  treated. 

No.  2 and  No.  3 are  developed  for  5 minutes  to  form 
the  positives,  so  as  to  give  good  printing  density.  Fix, 
wash,  and  dry,  and  use  for  printing  the  positives. 
Printing  plates  are  now  made  from  these  three  lantern 
plates  by  the  Pinatype  method  ( q.v .)  by  sensitising 
gelatine-coated  plates  in  a solution  of  1*25  per  cent,  of 
Potassium  Bichromate. 

Printing  the  Plates.  — These  sensitised  plates  are 
first  dried  in  the  dark,  and  are  then  placed,  film  to 
film,  with  the  No.  2 and  No.  3 lantern  plate  positives, 
and  exposed  to  light.  The  time  of  exposure  is  best 
judged  by  means  of  an  “ Akuret  ” actinometer.  One 
piece  of  ordinary  P.O.P.  paper  is  placed  in  the  “Akuret  ” 
(in  No.  9 for  the  red,  and  in  No.  12  for  the  yellow. 
When  No.  9 is  nearly  black  No.  2 plate  will  be  almost 
rightly  printed,  and  when  No.  12  is  nearly  black  No.  3 


THREE-PLATE  COLOUR  PRINTING  1 57 


plate  will  be  right.  After  washing  out  the  sensitiser, 
dissolve  out  the  silver  with  Farmer’s  Reducer,  wash 
well  and  place  No.  2 printer  in  the  pinatype  red  dye, 
and  No.  3 printer  in  the  pinatype  yellow  dye.  The 
free  dye  is  now  to  be  well  washed  out  of  the  print 
plate  with  running  water,  and  then  the  printing  paper 
is  applied  (red  preferably  first).  The  time  of  soaking 
in  the  dye-bath  depends  largely  on  the  depth  of  print- 
ing. For  the  red  plate  take  about  5 or  15  minutes. 
Examine  and  wash  off  the  free  dye  until  a vigorous 
image  is  made.  If  necessary,  the  plate  may  be  re-dyed. 
The  yellow  should  be  examined  after  one  or  two 
minutes,  so  as  not  to  overdo  it.  It  may,  however, 
take  10  or  15  minutes.  (The  yellow  image  will  be 
barely  perceptible.) 

If  the  Pinatype  method  be  adopted  for  the  red  and 
yellow,  place  the  print  plates  in  contact  with  the  Pina- 
type transfer  paper,  first  with  the  red  print  plate,  and 
then  with  the  yellow  one,  being  careful  to  keep  them 
in  register. 

If  a P.O.P.  print  is  to  form  the  base,  the  red  print 
must  be  brought  in  contact  with  the  P.O.P.  image — it 
can  be  seen  through  the  back  of  the  print  plate.  When 
in  register,  squeegee  and  allow  it  to  stand,  usually  for 
about  five  minutes,  but  you  may  judge  by  turning  up 
the  corner.  The  yellow  print  is  transferred  in  the 
same  way.  This  also  takes  about  five  minutes.  The 
enlarged  negative  from  which  the  P.O.P.  print  is  made 
by  contact  may  be  formed  as  follows : — A positive  is 
made  by  contact  with  No.  3 negative,  and  placed  in 
the  enlarging  apparatus  (glass  side  to  the  lens),  and 
a black-tone  lantern  plate  exposed  in  the  usual  way. 


158  PHOTOGRAPHY  IN  COLOURS 

This  paper  negative  should  be  rather  smaller  than  the 
positives,  so  as  to  allow  for  the  expansion  of  the  paper 
when  wet.  By  always  using  the  same  make  of  paper, 
the  amount  of  expansion  can  be  readily  judged.  With 
Gem  Dry-plate  Co.’s  P.O.P.  paper,  the  necessary  altera- 
tion can  be  secured  by  advancing  the  plate  J inch 
nearer  the  lens  than  the  position  in  which  all  the  posi- 
tives are  taken,  and  at  the  end  of  the  enlarger  place 
the  smaller  positive  ~q  inch  further  from  the  lens. 

The  red- yellow  print,  prepared  as  already  described, 
must  now  be  brought  in  contact  with  the  converted 
blue  plate.  The  register  can  readily  be  seen  through 
the  back.  If  the  balance  of  colour  is  correct,  squeegee, 
and  when  dry,  the  picture  may  be  viewed  through  the 
back  of  the  blue  glass.  The  plate  may  now  either 
remain  in  contact  with  the  print,  or  the  print  may  be 
stripped  off,  leaving  the  bare  glass — provided,  of  course, 
that  the  converted  blue  plate  has  been  taken  on  a 
stripping  plate,  the  glass  of  which  has  been  prepared 
before  coating  with  the  emulsion.  These  plates  can 
be  bought  at  any  of  the  large  houses.1 

§ 86.  Three=colour  Half-tone  Process. — It  is  in 
photo-mechanical  processes  that  three-colour  work  has 
found  its  greatest  application,  and  that  chiefly  in  the 
half-tone  process.  Large  numbers  of  colour  prints 
are  now  executed  in  this  manner.  The  colour  plates  in 
this  book  may  be  taken  as  examples,  and  if  they  are 

1 This  description  has  been  almost  entirely  taken  from  Mr. 
E.  T.  Butler’s  paper  read  before  the  Society  of  Colour  Photo- 
graphers on  January  26,  1911,  and  introduced  here  by  his  kind 
permission.  He  supplies  the  apparatus  direct  from  his  address, 
see  Table  26,  Appendix. 


THREE-PLATE  COLOUR  PRINTING  I 59 

examined  with  a magnifying  glass,  it  will  be  found  that 
they  consist  of  dots  of  the  same  three  printing  colours 
(yellow,  magenta  pink,  and  cyan -blue)  as  are  used  in 
the  other  “ subtractive  ” process  described  elsewhere. 
The  use  of  half-tone  printing  in  this  connection  has 
the  great  advantage,  that  once  the  planting  blocks  are 
prepared,  a practically  unlimited  number  of  prints  may 
be  obtained.  The  preparation  of  these  blocks,  however, 
involves  the  comparatively  complicated  process  of  half- 
tone block  making. 

As  it  was  originally  worked,  the  process  consisted  in 
first  preparing  the  usual  three  negatives  of  the  subject 
through  the  colour  filters,  exactly  as  has  been  described 
for  other  methods ; then  from  these  negatives  three 
transparent  positives  were  made  on  ordinary  photo- 
graphic plates.  From  this  point  the  operations  become 
those  of  preparing  half-tone  blocks,  for  the  positives 
are  each  in  turn  illuminated  from  behind,  and  a half- 
tone screen  negative  is  made  from  each.  In  the 
making  of  these  negatives  a transparent  screen  with 
two  series  of  ruled  opaque  lines  crossing  each  other  at 
right  angles  is  placed  in  front  of  the  photographic 
plate,  when  the  varying  tones  of  the  positives  become 
translated  in  the  negative  into  dots  of  varying  sizes. 
This  screen  must  not  be  confounded  with  the  “ colour 
screens  ” described  in  Ch.  VIII.  and  IX.  Such  negatives 
are  printed  on  to  copper  or  zinc  plates  coated  with  a thin 
film  of  bichromated  fish-glue,  and  then  washed  in  cold 
water.  This  dissolves  away  all  the  still  soluble  gelatine 
unaffected  by  the  light,  leaving  a positive  print  in 
insoluble  glue  which,  after  being  heated  to  harden  it, 
acts  as  a resist  to  the  etching  liquid  with  which  the  plate 


i6o 


PHOTOGRAPHY  IN  COLOURS 


is  afterwards  treated.  The  etched  metal  plate  when 
mounted  on  wood  or  metal,  “ type  high,”  is  called  a 
“ block,”  and  has  innumerable  dots  of  varying  sizes 
standing  up  to  a common  level.  This  can  be  printed 
like  type  on  an  ordinary  printing  press. 

This  form  of  the  process  is  still  in  use  for  much  work 
that  has  to  be  photographed  away  from  the  studio — for 
copying  pictures  in  galleries,  for  instance— but  by  em- 
ploying either  fine-grained  dry-plates  or  collodion  emul- 
sion, and  having  the  colour  filter  and  the  half-tone 
screen  both  in  position  at  the  same  time,  the  colour 
negative  can  also  become  a “ screen  ” negative,  and 
thus  the  number  of  photographic  operations  may  be 
reduced.  By  whichever  method  the  three  blocks  are 
produced,  they  are  printed  in  succession  on  smooth 
surfaced  paper,  superposed  in  absolute  register,  the  order 
being  usually  the  yellow  first,  then  the  red,  and  lastly 
the  blue.  The  yellow  ink,  being  the  most  opaque,  is 
printed  first,  and  the  blue,  the  most  transparent,  last,  as 
any  slight  opacity  in  the  second  or  third  printings 
would  tend  to  obscure  the  effect  of  the  inks  underneath. 
All  “ subtractive  ” methods  are  somewhat  at  a disad- 
vantage compared  with  screen-plate  and  other  additive 
methods  with  regard  to  their  reproduction  colours. 
While  there  is  no  great  difficulty  in  obtaining  the 
correct  red,  green,  and  blue  for  “ screen  ” plates,  it  is, 
however,  still  impossible  to  procure  quite  the  right  cyan- 
blue,  magenta-pink,  and  yellow  for  any  of  the  subtractive 
methods,  and  the  three-colour  half-tone  process  suffers 
most  from  this  drawback.  The  cyan-blue  ink  is 
usually  the  most  defective  ; it  rarely  absorbs  all  the 
red  light  which  it  should  do,  while  its  reflection  of 


THREE-PLATE  COLOUR  PRINTING  l6l 


green  light  is  never  all  that  could  be  desired.  The 
magenta-pink  ink  does  not  reflect  sufficient  blue  and 
violet  light.  The  yellow  ink  is  fairly  satisfactory. 
As  a result,  many  colours  are  incorrectly  rendered, 
greens  are  too  dark,  purples  and  greys  too  red,  and 
reds  often  too  orange,  and  the  correct  hues  have  to  be 
recovered  by  locally  etching  the  blocks ; “ fine  etching  ” 
as  it  is  called. 

In  spite  of  this  disadvantage  great  strides  have  been 
made  with  the  process,  and  much  good  work  is  done 
by  it.  The  comparative  inexpensiveness  of  half-tone 
printing  is  also  greatly  in  its  favour. 

§ 87. — Collotype  Colour  Process. — Collotype  is  a 
printing  process  similar  in  many  respects  to  lithographic 
printing.  The  prints  are  obtained  in  printer’s  ink 
from  a reticulated  gelatine  surface.  Sheets  of  thick  plate 
glass  are  coated  very  thinly  with  a solution  of  gelatine 
and  bichromate  of  potash.  These  are  dried  level  in  an 
oven  or  large  heated  drying-box,  at  a temperature  too 
high  to  allow  the  emulsion  to  set.  When  cool  they  are 
exposed  under  the  negatives,  and  are  afterwards 
washed  in  cold  water  until  the  bichromate  is  entirely 
removed.  The  sheets  of  plate  glass  are  then  stood  up 
to  dry.  When  dry  the  image,  looking  similar  to  a 
steel  engraving  before  it  is  inked  up,  can  be  seen  on  the 
plate.  The  plate  is  then  soaked  in  glycerine  and  water, 
and  this  is  termed  in  the  trade  “etching”  it.  After 
about  half  an  hour’s  soaking  the  solution  is  mopped  off 
with  a dry  cloth. 

It  is  easy  to  understand  how  the  glycerine  solution 
will  penetrate  freely  through  the  soft  high  lights  of  the 
image,  while  the  shadows  and  half  tones,  hardened  by 

M 


162 


PHOTOGRAPHY  IN  COLOURS 


the  action  of  light  on  the  bichromate,  absorb  it  less 
readily,  or  not  at  all,  in  proportion  to  the  amount  of 
light  received,  so  that  the  image  consists  of  varying 
degrees  of  moisture  and  dryness.  When  the  inked 
(lithographic)  roller  is  passed  over  the  surface,  the 
greasy  ink  adheres  freely  to  the  dry  shadows,  but  is 
refused  by  the  moistened  high  lights.  The  reticulation 
of  the  gelatine  surface  gives  an  extremely  fine  grain 
over  the  plate,  invisible  to  the  naked  eye. 

This  method  of  printing  can  be  done  on  almost  any 
variety  of  paper  suitable  for  general  printing,  and  has 
the  great  advantage  over  half-tone  printing  blocks 
etched  upon  copper,  or  other  metal,  in  that  an 
artificially  coated  paper  with  a polished  surface  is  not 
essential  for  the  best  results.  The  printing  is  done  on 
a “ scraper”  press,  not  unlike  a lithographic  press. 

Since  a print  can  be  taken  in  this  way  with  a litho- 
graphic ink  of  any  colour,  all  that  is  required  will  be 
to  make  three  collotype  plates  from  the  three  negatives 
in  the  way  already  described ; then  to  ink  each  of  them 
over  with  one  of  the  three  complementary  printing 
colours,  and  finally  to  take  a print  on  a single  sheet  of 
paper  from  all  three  plates  successively,  taking  care 
that  the  three  impressions  lie  in  exact  superposition. 

In  practice  two  main  and  several  minor  difficulties 
arise.  In  any  process  of  three-colour  printing  the 
balance  of  the  respective  printings  must  be  perfectly 
maintained  in  all  the  prints  taken.  Collotype  plates 
are  peculiarly  liable  to  variation  with  changes  in 
humidity ; so  that  care  has  to  be  exercised  to  prevent 
the  weather  from  affecting  the  conditions  of  the 
printing  room. 


THREE-PLATE  COLOUR  PRINTING  1 63 


The  variable  climate  of  England  can  scarcely  be 
said  to  favour  the  process.  Then  from  its  very  nature 
there  is  but  little  opportunity  of  doing  any  local  work 
on  the  plates  to  remedy  defects,  so  that  it  is  very 
necessary  that  both  filters  and  inks  should  be  as 
perfect  as  possible,  and  whatever  adjustments  appear 
to  be  requisite  must  be  done  on  the  negative  before 
making  the  print  on  the  chromated  gelatine.  The 
inks  of  the  primary  colours  also  do  not  work  so 
kindly  on  the  rollers  as  does  black  ink,  and  therefore 
it  is  difficult  to  print  evenly  with  them.  The  yellow 
tends  to  clog  up  the  shadows.  The  red  is  too  greasy 
and  gives  harsh  contrasts.  The  blue  is  fairly  easy  to 
work  with. 

The  other  difficulty  is  fundamental.  We  refer  to  the 
superposing  of  the  whole  image  of  each  colour.  Of 
course,  each  of  the  three  negatives  must  have  been 
correctly  exposed,  but  even  then  the  yellow  if  printed 
first  is  apt  to  be  overwhelmed,  and  the  blue  on  the  top 
will  have  become  unduly  prominent. 

Moreover,  considerable  experience  is  necessary  in 
order  to  obtain  perfect  uniformity  when  any  consider- 
able number  of  copies  are  being  printed.  It  is  often 
desirable  to  print  a fourth  impression  made  from  a 
collotype  plate  obtained  from  a fourth  negative,  or 
sometimes  the  “ yellow  ” plate  is  reprinted  in  a soft  grey. 

In  spite  of  its  difficulty,  some  very  fine  work  has 
been  recently  done  in  colour  by  this  process,  and  it 
has  an  assured  future  in  connection  with  colour 
photography. 

§ 88.  Sanger-Shepherd’s  Imbibition  Process.— 

This  is  a practical  method,  fairly  easy  of  application,  and 


164  PHOTOGRAPHY  IN  COLOURS 

somewhat  resembles  his  method  for  making  transparency 
pictures.  Three  negatives  are  first  taken  through  the 
colour  filters,  as  has  been  already  mentioned.  The 
positives  from  them  are  printed  upon  a special 
celluloid  film  coated  with  gelatine  containing  bromide 
of  silver,  sensitised  by  immersion  in  the  sensitising 
bath  of  potassium  bichromate  for  three  minutes  and 
dried  in  the  dark  room. 

The  prints  are  made  upon  the  film  by  printing  through 
the  celluloid — the  celluloid  side  being  placed  in  contact 
with  the  film  side  of  the  negative  and  exposed  to  day- 
light until,  on  examination  in  weak  light,  all  the 
details  are  visible  on  the  film  as  a brownish-yellow 
print,  very  similar  in  appearance  to  an  undeveloped 
platinotype  print.  The  printed  film  is  immersed  in 
warm  water,  and  in  a few  minutes  the  unaltered 
gelatine  dissolves  away,  leaving  a perfect  white  image 
full  of  detail  attached  to  the  celluloid  base.  The  print 
is  next  fixed  in  ordinary  clean  hyposulphite  of  soda 
solution  until  the  white  bromide  of  silver  dissolves, 
leaving  a transparent,  low  relief  in  clear  gelatine. 
After  washing  in  water  for  ten  minutes,  the  prints  are 
ready  for  staining  up.  The  print  from  the  green  filter 
negative  is  stained  up  in  the  pink  bath,  and  the  print 
from  the  blue-violet  filter  negative  is  stained  up  in  the 
yellow  bath — the  staining  being  stopped  as  soon  as  the 
two  prints,  when  held  over  the  greenish-blue  print,  give 
neutral  tints  in  the  grey  shadows  of  the  picture. 
Should  one  of  the  positives  be  accidentally  overstained 
it  may  easily  be  reduced  by  merely  soaking  in  clean 
water.  They  are  then  successively  squeegeed  on  to  a 
piece  of  paper  coated  with  a thin  layer  of  gelatine. 


THREE-PLATE  COLOUR  PRINTING  1 65 

This  absorbs  the  dye  from  the  relief  surface  of  the 
hardened  gelatine.  The  gelatinised  paper  is  first  well 
soaked  in  water  and  spread  over  a glass  plate,  coated 
side  uppermost.  Then  the  pink- dyed  positive  (from 
the  negative  taken  through  the  green  screen)  is 
squeegeed  on  to  the  gelatine  paper  until  the  whole  of 
the  colour  has  been  taken  up  by  it.  In  the  same  way 
the  yellow-dyed  positive  (from  the  negative  taken 
through  the  blue  screen)  is  carefully  adjusted  in 
register  on  to  the  pink  impression,  and  squeegeed  down 
on  to  it.  Lastly,  the  blue-dyed  positive  is  squeegeed 
on  to  the  pink  and  yellow  image,  which  is  kept  wet  to 
get  an  even  impression.  If  any  one  of  the  colours  is 
too  weak,  the  printing  plate  for  that  colour  may  be  re- 
dyed and  used  again.  Thus  a paper  print  is  obtained 
on  which  an  image  built  up  of  three  colours  is  impressed. 
This  may  be  squeegeed  on  to  ground  glass  or  polished 
glass,  according  as  to  whether  a matt  or  glossy  surface 
is  desired.  The  print  is  now  finished,  and  if  the  process 
has  been  correctly  carried  out,  especially  the  correct 
exposures  in  the  first  instance,  the  result  will  be  an 
extremely  charming  effect  of  colour.  The  skies  are 
often  very  fine,  indeed,  much  superior  to  autochromes, 
as  the  proper  rendering  of  the  sky  is  the  chief  defect 
in  the  starch-grain  method.  According  to  Sanger  - 
Shepherd,  the  following  are  the  chief  sources  of 
failure,  with  their  remedies  : — 

The  'printing  plates  are  liable  to  stick  to  the  paper , 
because  the  paper  has  not  been  soaked  long  enough 
before  use.  It  should  be  soaked  in  clean,  cold  water 
for  at  least  ten  minutes. 

The  printing  plates  take  up  the  colour  all  over  when 


1 66  PHOTOGRAPHY  IN  COLOURS 

immersed  in  the  colour  bath.  This  is  owing  to  an 
over-printed  relief ; the  relief  must  be  thin.  The  best 
results  are  obtained  by  slow  development  in  water  at 
100°  F.  to  105°  F.  It  is  because  of  the  necessity 
for  using  a very  low  relief  that  a thin  negative  is 
recommended. 

Dark,  muddy  prints. — This  arises  from  printing  plates 
being  stained  too  deeply,  or  from  the  relief  being  over- 
printed. 

Blurred  prints. — This  fault  is  due  to  the  paper  being 
too  wet,  or  because  too  long  time  has  been  taken  in 
the  transfer,  owing  to  an  unsuitable  relief.  With  a 
correctly  printed  relief  the  whole  of  the  ink  should  be 
transferred  to  the  gelatinised  paper  within  five  minutes. 
The  finished  print  should  be  at  once  pressed,  surface 
dry,  between  clean  blotters,  and  pinned  up  to  dry  in 
a current  of  air. 

Full  detailed  instructions  are  sent  out  by  Sanger- 
Shepherd  & Co.,  along  with  all  the  materials  necessary 
for  carrying  out  their  process. 

§ 89.  The  Pinatype  Process. — This  method,  in- 
vented by  Dr.  E.  Konig,  has  considerably  grown  in 
public  favour  of  late,  and  is  well  able  to  hold  its  own 
among  competitors.  The  process,  like  Sanger-Shep- 
herd’s,  depends  on  the  selective  action  of  certain  dyes 
on  the  gelatine.  Thus,  supposing  the  three  gelatine 
bichromate  printing  plates  have  been  prepared  as  in 
the  last  process,  then  the  parts  exposed  to  light  will 
be  hardened,  the  rest  remaining  soft.  Now,  it  has 
been  found  that  dyes  may  be  classified  thus — 

1.  Those  (and  they  are  the  majority)  which  stain 
the  whole  surface,  either  uniformly  or  partly,  by  a 


THREE-PLATE  COLOUR  PRINTING  1 67 

selective  action ; the  dye  being  in  some  cases  removed 
by  the  water,  in  other  cases  remaining  fixed. 

2.  Those  dyes  which  stain  the  hardened  parts  of  the 
gelatine  more  than  the  nnhardened  parts,  since  they 
enter  into  composition  with  the  hard  (light-impressed) 
parts  which  contain  oxides  of  chromium. 

3.  A few  dyes  exist  which  do  not  touch  the  hard 
gelatine,  but  stain  the  (unacted  on)  soft  gelatine.  Such 
stains  are  called  pinatype  colours,  and  they  constitute 
the  dyes  used  in  this  process. 

It  will  thus  be  seen  that  the  pinatype  method  is  the 
reverse  of  the  Sanger-Shepherd,  since  the  latter  depends 
on  dyes  which  adhere  to  the  raised  hard  gelatine  and 
come  off  on  to  the  paper. 

Pinatype  colours  should  possess  the  following 
properties : — 

1.  They  must  be  fairly  soluble  in  cold  water. 

2.  They  must  stain  the  soft  gelatine  strongly,  and 
hardly  touch  the  hard  parts. 

3.  They  must  be  fixed  dyes,  and  incapable  of  being 
washed  out. 

4.  They  must  readily  stain  the  paper  brought  in 
contact. 

5.  The  picture  must  retain  its  detail  and  sharpness 
after  drying,  and  must  not  suffer  from  prolonged 
washing. 

6.  Lastly,  the  colours  must  not  be  liable  to  fade. 

Fortunately  all  these  properties  can  be  found  among 

the  red,  yellow,  and  blue  dyes. 

It  may  also  be  noticed  that  since  these  dyes  do  not 
stain  the  light-impressed  gelatine,  but  only  the  parts 
unacted  on,  the  pinatype  print  will  be  a facsimile  of 


1 68 


PHOTOGRAPHY  IN  COLOURS 


the  original  negative.  In  other  words,  the  original 
negative  taken  through  the  colour- screen  is  reproduced 
by  the  printing  plate  exactly  as  in  the  Sanger- Shepherd 
process,  but  with  this  difference : By  the  latter 
method  the  print  is  made  from  the  light-hardened 
gelatine  which  receives  the  dye,  whereas  by  the 
pinatype  process  it  is  the  unchanged  gelatine  which 
takes  up  and  transfers  the  colour.  In  order,  therefore, 
to  make  a positive  print,  our  bichromated  printing 
plate  must  be  made  from  a transparency  (diapositive), 
and  not  from  a negative,  as  in  the  other  method. 

To  sum  up,  the  pinatype  process  consists  of  five 
stages. 

1.  Making  the  Negatives. — Three  negatives  of  the 
subject  are  taken  through  their  respective  screens. 

2.  Copying  the  Negatives.  — Three  transparencies 
(diapositives)  are  made  from  the  negatives  on  a fine-grain 
emulsion,  such  as  is  used  for  lantern  slides.  These 
can  be  made  to  any  size,  so  that  if  an  enlargement  or 
reduced  print  be  wanted,  the  diapositives  can  be  made 
to  the  size  required  in  the  print.  The  qualities  of  a 
lantern  slide  are  transparency,  brilliancy,  and  contrast, 
but  the  pinatype  diapositives  should  be  soft,  without 
any  great  amount  of  density  anywhere.  This  quality 
can  be  readily  obtained  by  giving  a full  exposure,  and 
using  a diluted  developer. 

3.  Transferring  the  Image  to  the  Printing  Plate. — ■ 
Prom  the  diapositives  three  printing  plates  are  made. 
These  are  glass  plates  thinly  coated  with  gelatine  and 
sensitised  in  a per  cent,  solution  of  bichromate  of 
potash  (15  grains  to  2 oz.  of  water),  dried  in  the  dark 
(6  to  8 hours),  and  then  successively  exposed  behind  the 


THREE-PLATE  COLOUR  PRINTING  1 69 

diapositives.  Each  plate  should  be  marked  B,  E,  or  Y 
in  the  corner,  to  indicate  the  colour  to  be  used.  The 
sensitising  solution  must  be  kept  cool  (60°  to  65°  E.), 
and  the  time  for  each  printing  regulated  by  a 
photometer  or  actinometer  (Warnerke’s  or  Sanger- 
Shepherd’s).  It  is  about  the  same  as  for  collodion 
P.O.P.  The  image  appears  faintly  drawn  on  a yellow 
ground.  The  plates  should  be  well  washed  until  all 
the  yellow  has  disappeared  from  the  water.  They  are 
then  dried,  and  are  ready  for  use  at  any  time. 

4.  Dyeing  the  plates. — Three  baths  are  to  be  made. 
A blue  bath  of  10  tablets  pinatype  blue  to  9 oz.  of  water 
for  the  plate  from  the  red  screen  negative  (immerse  for 
15  to  20  min.) ; a red  bath  of  10  tablets  pinatype  red  to 
1 drachm  of  ammonia  *880  and  9 oz.  of  water  for 
the  plate  from  the  green  negative  (immerse  for  10 
to  15  min.) ; and  a yellow  bath  10  tablets  of  pina- 
type yellow  to  7 oz.  hot  water  (immerse  for  half 
an  hour). 

5.  Printing  the  picture  on  paper. — A sheet  of  transfer 
paper  is  soaked  in  water  until  it  expands  no  longer. 
It  is  then  gently  and  evenly  squeegeed  down  on  to  the 
blue-dyed  plate,  which  is  taken  wet  from  the  bath.  A 
piece  of  oiled  paper  is  laid  over  the  print  to  enable  the 
roller  squeegee  to  run  smoothly.  The  progress  of  the 
transfer  of  the  colour  to  the  print  must  be  watched 
by  turning  up  one  of  the  corners  from  time  to  time. 
On  an  average  about  ten  to  fifteen  minutes  will  suffice. 
The  blue  print  is  then  removed  and  transferred  to  the 
red-dyed  plate.  In  this  case  it  is  well  to  place  a thin 
transparent  sheet  of  celluloid  between  the  two,  and  as 
soon  as  the  two  are  in  register,  to  hold  the  top  of  the 


170  PHOTOGRAPHY  IN  COLOURS 


print  firmly  and  slip  the  celluloid  from  underneath, 
and  then  to  squeegee  as  before.  This  precaution  is 
necessary  to  prevent  the  transfer  of  colour  before 
register  is  secured.  In  the  case  of  the  yellow  dye  this 
is  not  necessary,  as  it  acts  more  slowly. 

Lastly,  the  print  now  dyed  with  blue  and  red  is 
squeegeed  down  upon  the  yellow-dyed  plate.  The 
order  is  therefore  Blue -red-yellow,  but  you  may  make 
it  Red-blue-yellow,  or  even  Blue -yellow-red,  but  only 
experience  will  teach  you  which  is  best  for  each  case. 
If  any  of  the  prints  have  been  dyed  too  deeply,  the 
colour  may  be  thinned  down  by  squeegeeing  them  on 
to  a piece  of  paper  coated  with  gelatine  until  sufficient 
colour  has  been  abstracted.  In  the  same  way  an 
unfixed  print  which  is  too  weak  may  be  reinforced 
by  squeegeeing  it  on  to  its  printing  plate.  Retouching 
may  be  done  on  any  of  the  wet  prints  with  a brush 
soaked  in  the  dye. 

One  great  advantage  of  this  process  lies  in  the  fact 
that  the  three  impressions  of  colour  are  superposed 
on  the  single  support,  and  not  on  separate  gelatine 
layers  which  require  to  be  accurately  placed  in  register. 
The  weakest  part  of  the  picture  lies  in  the  blues, 
which  are  apt  to  become  too  red  owing  to  the  varying 
effect  of  the  green  filter. 

§ 90.  The  Colour  Carbon  and  other  Processes. — 

Many  other  beautiful  and  useful  processes  exist,  such 
as  the  Rotary  Co.’s  Stripping  Pigment  Films,  in 
which  the  printing  is  done  through  thin  sheets  of 
celluloid  and  each  developed  pigment  image  is  in  turn 
transferred  to  a piece  of  single  transfer  paper.  The 
Hesekiel-Selle  carbon  process  and  the  Perscheid  screen 


THREE-PLATE  COLOUR  PRINTING  1 7 I 


process  may  also  be  mentioned  in  this  connection,  but 
it  is  beyond  the  scope  of  this  work  to  enter  into  details 
respecting  them,  since  they  are  quite  unsuitable  for  the 
amateur  by  reason  of  the  apparatus  and  skill  required. 

§ 90a.  Raydex  Colour  Process. — This  is  a new 
colour  printing  process  which  seems  to  be  rapidly 
gaining  in  favour,  as  it  is  easy  to  work,  reliable,  and 
permanent.  In  brief,  the  process  is  as  follows  : Three 
separate  negatives  are  made  behind  red,  blue  and  green 
filters,  respectively.  From  these  negatives  bromide 
prints  are  made  which,  when  washed  and  fixed,  are 
soaked  in  water  and  laid  in  contact  with  Baydex  colour 
sheets  of  the  complementary  colours  for  twenty  minutes 
The  colour  sheets  are  now  stripped  off  the  bromide  prints, 
and  each  is  squeegeed  on  to  a celluloid  plate  and  im- 
mersed in  hot  water  until  the  unacted- on  colour  is 
dissolved  off.  The  three-colour  prints  are  now  dried, 
and  then  one  is  placed  on  a paper  support  under  water 
and  again  dried  when  the  colour  picture  on  the  support 
is  stripped  off.  Then  two  of  the  prints  are  carefully 
placed  in  register  under  water,  taken  out  and  dried, 
and,  finally,  the  third  colour  print  is  adjusted  in  register 
on  the  other  two.  The  three  prints  now  adherent  are 
again  dried  and  the  celluloid  stripped  off.  The  finished 
picture  is  then  trimmed  and  mounted,  and  can  be 
framed  and  hung  up  on  the  wall.  The  initial  outlay 
is  comparatively  small,  since  any  ordinary  camera  and 
lens  will  suffice,  although  (as  previously  pointed  out)  an 
astigmat  lens  is  preferable.  The  only  provisos  being 
that  the  camera  should  not  be  of  the  roll  film  type,  and 
further,  that  the  plate  holders  should  be  of  the  hinge 
type,  and  provided  with  springs  if  the  filters  are  to  be 


PHOTOGRAPHY  IN  COLOURS 


I 72 

placed  in  contact  with  the  plate.  A long  slide  carrying 
the  three  plates  side  by  side  is  to  be  preferred,  but 
double  backs  may  be  used,  the  slide  being  turned  round 
for  the  second  exposure,  and  a second  slide  employed 
for  the  third  exposure.  This  process  has  the  further 
advantage  that  a small  camera  may  be  used,  since 
enlargements  may  be  made  from  the  original  negatives 
and  colour  prints  obtained  from  two  to  three  times  the 
size  of  the  negatives.  In  a good  light,  all  three  exposures 
will  only  occupy  about  12  to  15  seconds  with  a long 
slide,  and  a few  seconds  more  with  separate  slides. 

Details  of  the  Process. — The  Negative.  — First, 
three  negatives  are  made  on  panchromatic  plates 
through  three  colour  filters  exactly  as  in  the  Sanger- 
Shepherd  process  (see  p.  163).  The  filters  may 
either  be  cut  the  size  of  the  plates,  and  placed  in 
the  slide  in  contact  with  them,  or  they  may  be 
placed  in  holders  and  fitted  to  the  lens  in  front  or 
behind.  Some  photographers  prefer  coloured  glass 
filters ; others,  thin  filters  made  of  dyed  gelatine. 
Any  of  the  panchromatic  plates  on  the  market  will  do. 
They  should  preferably  be  backed  to  avoid  halation. 
These  should  be  marked  E,  G,  and  B to  correspond 
with  the  filters.  As  regards  the  time  of  exposure 
behind  the  “blue”  plate,  give  with  F/8  a quarter  of 
the  time  that  the  paper  of  Watkin’s  or  Wynne’s  meter 
takes  to  match  the  tint.  Thus,  if  the  Actinometer  takes 
10  secs,  to  turn  dark,  give  about  two  and  a half  secs. 
Each  box  of  Wratten’s  plates  gives  the  ratio  of  the 
exposures  for  the  three  colours.  All  three  plates 
should  be  developed  in  the  same  dish,  and  at  the 
same  time  in  deep  Yirida  light. 


THREE-PLATE  COLOUR  PRINTING  I 73 

The  Bromide  Prints. — From  each  of  these  negatives 
when  fixed,  washed  and  dried,  a print  is  made  on 
Raydex  Bromide  paper.  Each  should  be  marked  R,  G, 
B,  in  the  corner,  to  correspond  to  the  negatives  taken 
under  the  red,  green,  and  blue  filters,  and  care  should 
be  taken  to  cut  them  all  from  the  paper  the  same  way 
of  the  grain,  so  as  to  secure  equal  expansion,  and 
consequently  correct  register  when  soaked  in  water. 

As  soon  as  the  papers  are  soaked,  place  them  side 
by  side  in  a clean  porcelain  dish,  and  develop  with  any 
good  developer.  The  Raydex  Company  recommend  their 
single  solution  developer,  diluted  1 in  20.  A weaker 
solution,  1 in  30  or  40,  is  slower,  but  allows  of  greater 
control.  This  weak  solution  is  advisable  in  the  case  of 
beginners,  as  if  development  is  too  rapid  a weak  image 
will  result.  The  prints  must  then  be  fixed  in  Hypo 
solution — an  acid  fixing  bath  is  to  be  preferred — in 
which  they  should  be  left  for  at  least  ten  minutes. 
They  are  then  removed  and  thoroughly  washed  free  of 
Hypo,  and  then  dried  for  future  use  ; or  when  half  dry 
they  may  be  placed  on  a chemically  clean  glass  plate, 
which  acts  as  a support — and  a colour  print  made 
directly  from  them. 

The  Colour  Prints. — While  the  bromide  prints  are 
half  dry  on  their  glass  supports,  cut  three  pieces  off 
the  Raydex  colour  sheets  a quarter  of  an  inch  larger 
than  the  Bromide  paper,  prepare  three  Raydex  trans- 
parent supports  by  rubbing  over  each  support  with  a 
few  drops  of  the  wax  solution  with  a soft  rag,  and 
then  polish  it  off  with  a fresh  clean  rag,  avoiding 
streaks.  Take  two  porcelain  dishes.  Fill  the  larger 
one  with  clean  water  and  place  the  smaller  one 


174  PHOTOGRAPHY  IN  COLOURS 


in  front  of  you  to  sensitise  the  colour  sheets  in. 
Next  place  the  three  colour  sheets  in  the  water,  and 
allow  them  to  soak  until  they  uncurl  and  flatten  out, 
which  they  will  do  in  a few  moments,  and  then  hang 
them  up  to  drain.  Or,  if  you  prefer  it,  you  may 
merely  sponge  over  the  backs  of  the  colour  sheets  with 
a wet  sponge,  taking  care  not  to  touch  the  colour  side. 
They  will  at  once  curl  up,  but  will  soon  flatten  down 
again. 

Now  proceed  to  develop  the  colour.  Measure  out 
one  dram  of  each  of  No.  1 and  No.  2 solutions  and  add 
to  them  an  ounce  of  water.  This  will  be  sufficient  to 
sensitise  three  quarter-plate  sheets — or  even  three 
half-plate  sheets  with  care.  Immerse  the  colour 
sheets  in  the  solution  for  about  two  minutes,  moving 
them  about  all  the  time.  Now  take  the  bromide  print 
marked  R (made  from  the  negative  taken  through  the 
red  screen)  in  the  one  hand,  and  in  the  other  hand 
lift  up  the  blue  sheet  from  the  bath,  drain  off  the 
excess  of  solution  and  slide  it  over  the  bromide  print 
still  adherent  to  the  glass  support  under  the  surface  of 
the  water  in  the  larger  dish,  taking  care  that  there  is 
a margin  of  colour  sheet  all  round  the  print.  Withdraw 
the  two  sheets  in  contact,  and  rapidly  squeegee  the  two 
sheets  together  so  as  to  get  rid  of  all  bubbles,  being 
careful  not  to  shift  the  papers,  as  chemical  action  is 
going  on  all  the  time,  and  any  movement  will  give 
rise  to  a double  image.  Next,  dry  the  back  of  the 
top  sheet  with  blotting  paper,  and  strip  both  papers 
off  together  with  a flat  knife  off  the  glass,  and  then  dab 
the  moisture  off  the  other  side.  Then  hang  up  the 
two  sheets  stuck  together  to  dry.  Leave  the  prints  in 


THREE-PLATE  COLOUR  PRINTING  I 75 


contact  for  twenty  minutes  so  as  to  allow  the  bromide 
paper  to  become  thoroughly  bleached.  Treat  the  other 
bromide  prints  in  the  same  way,  i.e.  place  the  “ green  ” 
Bromide  print  in  contact  with  the  red  colour  sheet,  and 
the  “ blue  ” print  in  contact  with  the  yellow  colour 
sheet.  Renew  the  water  in  the  big  dish  the  moment 
it  becomes  coloured.  Hang  up  the  three  combined 
sheets  which  have  been  roughly  dried  for  twenty 
minutes,  when  the  action  will  be  complete. 

It  is  worth  while  pointing  out  that  the  bromide 
prints,  after  they  have  fulfilled  their  purpose,  may  be 
washed  and  redeveloped,  and  again  used  for  a fresh 
set  of  prints.  Now,  take  a celluloid  sheet  previously 
cleaned  with  benzole  and  coated  with  waxing  solution. 
Strip  one  of  the  colour  sheets  off  the  bromide  paper 
(this  should  require  a slight  pull  to  detach,  and  not 
come  off  too  readily,  or  it  will  show  that  it  had  not 
been  dried  enough),  and,  after  dipping  it  for  a second 
in  water,  lower  the  centre  of  the  colour  sheet,  face 
downwards,  on  to  the  waxed  celluloid  surface,  and 
then  smooth  down  the  two  ends.  Squeegee  it  firmly 
in  contact  with  the  celluloid,  using  first  a flat  squeegee, 
so  as  to  exclude  all  air  bubbles.  Then  squeegee  again, 
under  blotting  paper,  with  the  roller  squeegee,  using 
several  pieces  of  blotting  paper,  so  as  to  dry  the  colour 
sheet  as  much  as  possible.  Treat  the  two  other  sheets 
in  the  same  way.  Then  proceed  to  develop. 

This  is  done  by  placing  the  three  sheets  in  a large 
dish  of  hot  water  at  about  110°  F.  or  115°  F.  until  the 
colours  begin  to  ooze  around  the  edge  of  the  paper. 
Now  strip  off  the  paper  support,  and  rock  the  plate  in 
the  water  until  all  the  superfluous,  unacted  colour  is 


Ij6  PHOTOGRAPHY  IN  COLOURS 

dissolved  off,  and  the  high  lights  are  quite  clear  and 
transparent.  As  soon  as  you  are  quite  sure  that  all 
the  unaffected  colour  is  dissolved  off,  rinse  in  a fresh 
dish  of  cold  water.  If  any  colour  remains  and  blocks 
out  the  high  lights,  pass  a smooth,  broad  camel’s  hair 
brush  over  the  surface,  taking  care  that  the  brush  is 
free  from  all  traces  of  grit.  Then  stand  the  print  up 
to  dry. 

Combining  the  Colour  Positives. — Single  Transfer. — 
We  have  now  three  prints — a blue,  a red,  and  a green 
one — attached  to  three  separate  sheets  of  celluloid. 
These  prints  have  now  to  be  brought  into  register. 
Soak  a piece  of  Eaydex  single  transfer  paper  in  water 
until  it  becomes  soft.  Dip  the  celluloid  plate  carrying 
the  yellow- coloured  image  into  water,  and  slide  the 
image  over  the  paper  support.  Remove  it  from  the 
water,  drain,  and  then  squeegee  into  contact,  using  a 
roller  squeegee  with  blotting  paper  between.  Brush 
over  the  blue  and  red  positives  with  a little  Raydex 
combining  solution  with  a broad  camel’s  hair  brush. 
Leave  them  to  dry  on  a flat  surface  out  of  the  dust. 
As  soon  as  the  yellow  positive  is  quite  dry,  strip  it  off 
the  support,  which  can  be  readily  done  by  bending  it 
slightly.  Then  remove  all  traces  of  wax  by  gently 
rubbing  the  surface  with  a soft  rag  moistened  with 
benzole.  Soak  in  water  along  with  the  blue  positive, 
and  roughly  register  them  under  the  water.  Then 
remove  and  lightly  squeegee,  and  slide  the  print  until 
the  two  are  in  perfect  register,  avoiding  all  pressure, 
so  as  to  prevent  the  two  surfaces  from  sticking  and 
tearing  until  they  are  precisely  in  apposition.  Allow 
them  to  dry  thoroughly,  and  detach  the  two  prints, 


THREE-PLATE  COLOUR  PRINTING  I 77 


now  firmly  adherent,  from  the  celluloid.  Eub  with 
benzole,  and  apply  the  red  print  in  the  same  way. 
When  the  final  celluloid  support  is  detached,  the  picture 
will  have  a fine  polished  surface.  If  a mat  surface  is 
desired,  clean  the  surface  with  a soft  handkerchief 
dipped  in  benzole,  and  then  dip  the  print  in  water  for 
a moment  and  allow  the  picture  to  dry  quite  flat.  The 
picture  is  now  ready  to  be  trimmed  and  mounted. 

The  picture  will  be  found  to  be  reversed  as  regards 
right  and  left.  This  is  immaterial  for  many  subjects, 
such  as  flowers  and  similar  objects,  but  whenever  it  is 
imperative  that  the  picture  should  be  the  right  way 
round  as  seen  by  the  eye,  the  original  negatives  must 
either  be  taken  with  the  glass  side  of  the  plate  facing 
the  lens,  or  the  double  transfer  method  must  be  used. 

Double  Transfer  Method. — Soak  a piece  of  Eaydex 
temporary  paper  support  in  water  for  a few  minutes, 
and  apply  it  to  the  blue  or  red  positive  as  previously 
described.  In  this  method  the  yellow  image  must  be 
laid  on  last,  and  the  combining  solution  should  be 
brushed  over  the  last  two  images  to  be  superimposed. 
As  soon  as  all  three  colour  positives  are  on  the  tem- 
porary support,  clean  with  benzole,  and  soak  in  water 
at  about  62°  F.  for  a minute  along  with  a piece  of 
Eaydex  final  support  a trifle  larger  than  the  picture. 
Now  remove  both  in  contact  and  squeegee,  and  place 
between  sheets  of  blotting  paper  under  a flat  weight 
for  ten  minutes. 

In  order  to  remove  the  temporary  support,  float  on 
the  surface  of  a dish  of  water  at  about  120°  F.,  avoid- 
ing air  bubbles,  taking  care  that  the  support  is  on  the 
top  and  kept  dry.  After  two  or  three  minutes,  turn 

N 


178  PHOTOGRAPHY  IN  COLOURS 

over  so  that  the  temporary  support  is  on  the  top,  and 
then  submerge  the  whole  under  the  water.  Gently 
remove  the  temporary  support,  and  wash  off  all  the 
soluble  gelatine.  The  picture  is  now  ready  to  be 
trimmed  and  mounted. 

If  the  beginner  fails  to  get  satisfactory  results  the 
Eaydex  Company  state  that  they  will  be  only  too 
pleased  to  put  him  on  the  right  road,  and  it  will  greatly 
facilitate  matters  if  the  prints  are  sent  for  inspection. 


CHAPTER  XI 


COLOUR  PRINTING  FROM  SINGLE-PLATE  TRANSPARENCIES 

§ 91.  Uto= color  Printing  from  Single = Plate  Trans = 
parencies. — The  reproduction  of  colour  transparencies 
on  to  paper  so  that  they  can  be  framed  on  a wall,  or 
pasted  in  an  album,  has  until  recently  only  been  effected 
by  making  three  separate  glass  negatives,  or  printing 
blocks,  and  then  either  superposing  transfer  films  stained 
with  the  complementary  colours,  or  by  direct  printing  of 
these  colours,  as  is  done  in  the  half-tone  colour  process. 
Such  methods  will  be  found  described  in  Chapter  X. 
Although  copies  of  remarkable  delicacy  and  beauty  can 
be  produced  by  these  methods,  they  all  require  three 
separate  negatives  to  be  made,  besides  entailing  a cer- 
tain amount  of  apparatus  coupled  with  a degree  of 
delicate  manipulation  which  can  only  be  obtained  by 
constant  practice.  Hence  any  method  by  which  a 
colour  transparency  can  be  directly  printed  on  a sheet 
of  prepared  paper  would  be  a great  desideratum.  In 
fact,  the  difficulty  of  reproducing  colour  transparencies 
has  been  the  main  cause  of  the  want  of  popularity  of 
colour  photography  among  amateurs. 

This  difficulty  has  at  length  been  more  or  less 
overcome  by  the  Bleach-out  Process  of  colour  print- 
ing, which,  although  very  far  from  perfect,  is  daily 
being  improved  upon.  It  has  at  length  reached  such 


i8o 


PHOTOGRAPHY  IN  COLOURS 


a stage,  that,  with  a suitable  colour  transparency,  very 
effective  and  faithful  copies  on  paper  are  reputed  to 
have  been  obtained  in  Europe. 

We  will  now  proceed  to  describe  the  methods  and 
the  principles  on  which  it  is  founded. 

§ 92.  The  Theory  of  the  Bleach  = out  Process. — It 
is  well  known  to  everybody  that  the  colours  of  wood 
and  textiles  change  under  the  prolonged  action  of  light. 
Wall-papers,  carpets,  watercolour  paintings,  etc.,  all 
tend  to  lose  the  brightness  of  their  colours,  and  partially 
bleach-out  in  sunlight.  Most  colouring  matters  (dyes) 
when  added  to  fabrics  are  not  permanent.  They  tend 
to  fade,  and  are  for  the  most  part  removable  by  wash- 
ing. A few  dyes  like  Indigo  and  Safflower  immediately 
impart  a permanent  colour  to  any  fabric  when  it  is 
dipped  or  boiled  in  the  dyeing  vat.  Hence  they  are 
called  substantive  colours.  Most  woollen  goods,  and  to 
a certain  extent  silks,  form  a permanent  compound 
with  dyes,  and  require  no  further  treatment ; but  the 
vast  majority  of  stuffs  require  a special  re-agent  called 
a mordant , which  will  form  an  insoluble  compound  both 
with  the  fabric  on  the  one  hand  and  with  the  dye  on 
the  other.  The  colours  which  require  such  treatment 
are  termed  adjective  colours. 

Alum,  Cream  of  Tartar,  Tannin,  and  the  oxides  of 
Iron  and  Tin  are  the  principal  mordants  employed. 

The  power  of  resisting  the  action  of  light  varies 
enormously  with  different  colouring  substances,  and 
their  resistance  depends  very  largely  on  the  colour  of 
the  light  which  attacks  them.  It  is  by  making  use  of 
these  facts  that  we  are  able  to  place  certain  colours  or 
pigments  on  to  the  paper  which  are  affected  by  different 


UTO-COLOR  PRINTING 


1 8 1 


coloured  lights,  and  at  the  same  time  are  quite  under 
control.  Of  course,  it  is  easy  enough  to  make  a direct 
copy  in  almost  any  single  colour,  but  it  is  a much  more 
complicated  problem  to  get  the  various  colours  to  re- 
produce themselves  on  the  same  sheet  of  paper  by 
selective  action  of  the  light.  The  direction  in  which 
the  solution  of  the  problem  may  be  sought  for  appears 
to  lie  in  the  bleaching-out  process. 

In  order  to  understand  the  theory,  we  may  as  well 
begin  by  explaining  what  is  meant  by  a pigment.  A 
pigment  is  a colouring  matter  or  fluid  which  permits 
a portion  of  the  rays  of  white  light  to  pass  through  it 
and  absorb  the  rest.  If  a pigment  in  a fluid  state  such 
as  water  or  oil  paint,  or  aniline  dye,  be  spread  over  a 
sheet  of  white  paper,  a certain  portion  of  the  light  will 
pass  through  the  layer  to  the  white  paper,  and  after 
being  reflected  will  again  pass  through  the  coloured 
layer,  and  give  rise  to  the  sensation  of  colour  in  the 
layer  in  question.  If  only  a small  amount  of  light  can 
pass  through  to  the  white  paper,  it  is  called  an  opaque 
colour.  Such,  for  example,  are  Chinese  White  and 
Naples  Yellow.  Such  a colour  is  said  to  have  body , and 
is  called  a tody  colour , or  opaque  colour,  because  it  hides 
the  colours  beneath  it.  If  a large  quantity  of  light 
passes  through,  it  is  called  a transparent  colour,  of  which 
most  of  the  aniline  dyes,  the  Umbers  and  Siennas,  are 
examples.  They  allow  the  subjacent  colours  to  shine 
through,  or  merely  modify  them.  Colours  made 
luminous  by  transmitted  light,  such  as  coloured  glasses, 
and  the  stained  glasses  used  for  windows,  as  well  as 
the  colours  of  the  spectrum,  are  all  good  examples  of 
transparent  colours. 


i82 


PHOTOGRAPHY  IN  COLOURS 


§ 93.  Permanent  and  Fugitive  Colours.  —When 
white  or  coloured  light-rays  are  absorbed,  they  may 
either  be  changed  into  heat,  without  altering  the  com- 
position of  the  pigment,  in  which  case  they  are  called 
permanent  or  fast  colours ; or  the  light-rays  may  alter  or 
split  up  the  molecules  of  the  colouring  matter,  in  which 
case  the  colour  will  be  altered,  or  may  even  entirely 
disappear.  These  pigments  are  called  sensitive  or 
fugitive  colouring  matters . 

If  the  rays  of  light  which  fall  on  the  surface  of  a 
body  are  scattered  diffusely,  such  a surface  will  appear 
white,  and  we  call  the  body  a white  one.  Snow,  milk, 
lime,  white  lead,  all  reflect  light  diffusely,  and  therefore 
convey  the  sensation  of  white ; but  if  a surface  reflect 
light  regularly,  so  as  to  form  an  image,  it  no  longer 
appears  white,  but  constitutes  a mirror,  and  will  have 
a metallic  lustre.  The  surface  of  still  water,  glass, 
quicksilver,  polished  metals  are  familiar  examples  of 
what  we  mean. 

If,  on  the  other  hand,  the  rays  of  light  are  nearly  all 
absorbed,  little  or  no  light  is  reflected,  and  the  surface 
of  the  body  appears  black.  As  we  have  already  ex- 
plained, no  substance  is  absolutely  white  or  black,  since 
the  whitest  body  known,  viz.  fresh  fallen  snow,  only 
reflects  about  85  per  cent,  or  90  per  cent,  of  the  light, 
and  the  blackest  material,  such  as  black  velvet,  powdered 
charcoal  or  soot,  reflects  about  1 per  cent,  of  the  light. 

Grothus,  writing  in  1819,  first  explained  the  theory 
of  the  action  of  light  on  coloured  bodies.  He  says, 
“ Coloured  light  seeks  to  destroy  in  bodies  upon  which 
it  acts  those  colours  which  are  opposed  to  its  own 
(i.e.  complementary  colours),  while  it  endeavours  to 


UTO-COLOR  PRINTING  1 83 

retain  its  own  or  another  analogous  colour.”  Thus, 
red  light  would  seek  to  discharge  its  complementary 
colour  blue-green,  while  retaining  the  orange-yellow; 
and  green  light  would  discharge  red,  while  retaining 
the  green.  In  other  words,  a coloured  body,  whether 
it  be  a fast  colour  or  one  sensitive  to  light,  is  not 
affected  by  light  of  its  own  colour,  since  it  either 
allows  all  rays  of  such  light  to  pass  through  it,  or 
else  reflects  them  away.  It  is  only  the  absorbed  rays 
which  decompose  or  destroy  the  colour.  Now,  a red 
coloured  body  does  not  absorb  red  rays,  but  it  will 
absorb  rays  of  other  colours.  If  this  coloured  body 
is  a fast  colour,  the  only  change  observed,  when  other 
than  red  rays  reach  it,  will  be  a rise  of  temperature, 
but  if  it  is  a body  sensitive  to  light  these  rays  will 
destroy  the  colour.  And  the  same  thing  will  happen 
in  the  case  of  any  other  colour.  Hence  we  may 
formulate  it  in  the  following  sentence,  which  Dr.  J.  H. 
Smith  calls  the  Bleach-out  Law : — 

“A  coloured  body  sensitive  to  light 
will  only  be  affected  by  the  light  it  absorbs, 
but  not  by  the  light  of  its  own  colour.” 
The  rays  which  are  absorbed  cause  a chemical 
change  in  the  molecules  of  the  sensitive  colouring 
matter  whereby  the  colour  is  destroyed.  In  other 
words,  the  colour  bleaches  out.  Now,  if  we  coat  a 
sheet  of  white  paper  with  a suitable  gelatine  emulsion 
containing  a purple-red  dye,  and  then  superpose  a 
yellow  dye,  and  lastly  a blue  dye  (taking  care  that 
these  are  the  proper  colours  and  in  the  right  propor- 
tions), we  shall  obtain  a black  mixture.  As  a matter 
of  fact,  the  result  is  rarely  a pure  black.  It  generally 


184  PHOTOGRAPHY  IN  COLOURS 

has  a greenish  or  greyish  tinge,  but  a good  black  is 
what  is  aimed  at,  and  with  the  increased  experience 
which  the  manufacturers  of  printing-out  colour  papers 
are  gradually  obtaining,  the  colour  is  approaching  nearer 
and  nearer  to  an  ideal  black.  Such  a three-coloured 
emulsion  layer  on  a white  backing  forms  the  basis  of 
a printing-out  colour  paper. 

The  Effect  of  Coloured  Lights  on  the  Payer . — Let  us 
suppose  that  a copy  on  paper  is  to  be  made  of  an 
Autochrome  Transparency.  Instead  of  a complicated 
landscape,  let  us,  for  the  sake  of  simplicity,  suppose 
that  the  transparency  consists  of  six  stripes  or  squares, 
A,  B,  C,  D,  E,  E (Fig.  25),  of  the  following  colours : 
Black,  Blue,  Red,  Green,  Yellow,  and  White.  A sheet 
of  the  sensitive  pigment  paper  is  placed  in  a printing 
frame  with  its  colour  (Black)  surface  in  contact  with 
the  film  side  of  a colour  transparency  (or  with  a piece 
of  thin  celluloid  between  the  two  in  order  to  prevent 
the  surfaces  sticking),  and  then  put  in  the  sunlight. 
What  happens  is  this.  At  A the  Black  stripe  blocks 
out  all  the  light,  so  that  no  action  takes  place,  and  the 
black  surface  of  the  paper  remains  unaltered. 

At  B the  Blue  stripe  allows  mainly  the  blue  rays  to 
pass  through.  According  to  the  Bleach-out  law,  the 
Blue  layer  will  be  unaffected,  since  the  Blue  rays  are 
not  absorbed,  but  pass  through  it ; but  the  light  will 
be  absorbed  by  the  yellow  and  red  pigments,  and  they 
will  be  destroyed  and  bleached  out,  so  that  ultimately 
when  the  action  is  completed,  all  the  light  which  passes 
through  the  blue  layer  will  reach  the  White  surface  of 
the  paper,  and  be  reflected  through  the  Blue  layer. 

At  C the  Red  stripe  allows  the  Red  light  to  penetrate 


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185 


the  Blue,  Yellow,  and  Red  pigments.  The  green  light 
will  only  be  absorbed  to  a small  extent  by  the  Blue 
pigment,  since  the  blue  and  green  waves  overlap  to 
a considerable  extent,  forming  a greenish- blue  or 
bluish-green  mixture.  For  the  same  reason  the  green 
light  will  only  act  in  a feeble  manner  on  the  Yellow 
pigment,  because  the  Yellow  and  Green  overlap,  form- 
ing a greenish  - yellow  or  yellowish  - green  mixture, 
according  to  the  nearness  of  the  waves  to  the  red 
or  the  blue  end  of  the  spectrum  respectively.  But 
the  green  rays  will  bleach  out  the  Red  pigment  almost 
entirely,  so  that  we  have  a Yellow  and  a Blue  layer  left 
practically  intact,  and  these  two  together  make  a green 
colour. 

At  E the  Yellow  stripe  will  permit  Yellow  rays  to 
pass,  and  these  will  bleach  out  the  Blue  and  the  Red 
so  that  only  the  Yellow  layer  is  left. 


Fig.  25. — Diagram  showing  the  action  of  light  on  the  emulsion 
colour  layer.  Three  separate  colour  layers  are  shown,  as 
occurs  in  the  Szczepanik  paper,  and  not  a single  layer  of 
mixed  colours,  as  is  the  case  in  the  Uto  paper.  This  is 
done  for  the  sake  of  clearness,  but  the  action  is  identical  in 
both  papers. 


At  F we  have  a White  or  clear  strip  of  glass  left. 
In  this  case  nearly  all  the  rays  which  compose  the 
whole  spectrum  pass  through,  so  that  all  the  three 
layers  are  acted  upon.  If  they  are  all  acted  upon 
equally,  they  will  all  be  bleached  out  together,  and  the 


1 86  PHOTOGRAPHY  IN  COLOURS 


White  of  the  paper  will  shine  through  the  gelatine  film 
unopposed,  and  the  stripe  will  appear  White.  This 
White  is  formed  by  a subtractive  process,  i.e.  by  removal 
of  all  the  superimposed  colours,  and  its  appearance  is 
due  to  exactly  the  opposite  process  to  the  White  seen 
in  a diapositive  (transparency),  for  in  this  latter  case 
the  White  is  formed  by  the  combination  of  the  three 
colours  (additive  process). 

If,  therefore,  the  pigments  have  been  perfectly 
chosen,  and  have  been  acted  on  by  the  light  so  as  to 
harmonise  with  the  exact  colours  of  the  stripes,  a perfect 
facsimile  of  the  original  ought  to  be  produced.  But, 
for  obvious  reasons,  an  exact  copy  can  only  be  approxi- 
mated, even  with  the  most  perfectly  chosen  pigments. 
For  even  if  the  pigments  were  right,  it  would  always 
be  impossible  to  regulate  the  light,  so  that  the  bleaching 
action  can  be  stopped  exactly  at  the  right  moment. 

Up  to  the  present  time  the  writer  has  never  once 
succeeded  in  getting  a satisfactory  print  from  any 
negative  by  this  method.  Perhaps  the  climate  of  S. 
Africa  is  unsuitable  by  reason  of  the  extreme  dryness. 

§ 94.  Details  concerning  the  Bleach = out  Pro= 
cess. — Having  explained  the  principle  of  the  Bleach- 
out  process,  we  will  now  consider  it  in  detail. 

Difficulties  to  be  Overcome  in  Procuring  Suitable  Dyes . 
— It  is  extremely  difficult  to  find  dyes  which  possess 
all  the  requisite  properties  for  producing  a suitable 
bleaching-out  paper.  For  example,  (1)  they  must  have 
no  chemical  effect  on  one  another,  and  (2)  they  must 
bleach  out  equally.  If  one  of  them  bleach  out  quicker 
than  the  other  two,  it  is  necessary  to  find  some  agent 
or  compound  which  has  to  be  added  in  order  to  retard 


UTO-COLOR  PRINTING 


1 8? 

its  action  to  the  requisite  degree.  (3)  After  the  colours 
have  been  bleached  out  by  the  light  in  the  parts  which 
represent  the  picture,  those  that  remain  must  be  capable 
of  fixation  so  that  the  light  will  have  no  further  action. 
(4)  Since  highly  sensitive  dyes  cannot  be  permanently 
fixed,  it  has  been  found  necessary  to  employ  less 
sensitive  dyes,  in  order  to  secure  the  essential  property 
of  stability  on  fixation.  (5)  These  moderately  sensitive 
dyes  must  be  capable  of  being  made  temporarily  highly 
sensitive,  and  then  of  being  rendered  less  sensitive 
afterwards.  (6)  Lastly,  the  dyes  must  not  only  possess 
all  these  properties,  but  they  must  have  the  right  shades 
or  tones  of  Red,  Yellow,  and  Blue,  so  that  together 
they  will  produce  Black,  Greys,  Browns,  Greens,  and 
Purples  of  the  proper  tones,  so  as  to  give  correctly 
balanced  mixtures.  It  will,  therefore,  be  readily  seen 
how  extremely  complicated  the  problem  is,  and  what 
enormous  difficulties  have  to  be  overcome,  in  order  to 
produce  ideal  light-sensitive  dyes. 

Supposing  that  we  have  the  right  dyes,  we  have 
then  to  mix  them  and  dissolve  them  equally  and 
thoroughly  in  an  emulsion  of  either  gelatine  or  collodion. 
Such  an  emulsion  must  have  no  chemical  or  injurious 
action  on  the  dyes,  and  should  permanently  retain  a 
certain  amount  of  moisture,  because  moisture  favours 
oxidation,  and  therefore  a bleaching-out  action. 

Now  we  may  adopt  one  of  two  methods.  We  may 
either  mix  all  three  colours,  red,  yellow,  and  blue  dyes 
with  the  gelatine  or  collodion,  and  then  spread  the 
mixture  over  the  paper,  as  is  done  in  the  Uto-colour 
process ; or  we  may  coat  the  paper  with  three  layers, 
each  holding  its  respective  dye,  as  is  employed  by 


1 88 


PHO T 0 GRA PHY  IN  COLOURS 


Szczepanik  in  his  three-layer  bleach-out  paper.  Each 
process  has  some  advantages  and  some  drawbacks. 
Both  answer  the  purpose  fairly  satisfactorily,  in  fact, 
the  single  layer  (Uto-color)  process  differs  more  in 
theory  than  in  practice,  because,  so  long  as  the  emulsion 
is  wet  the  colours  gravitate,  and  thus  arrange  them- 
selves into  layers  according  to  their  solubility.  Thus 
the  red  particles  which  are  less  soluble  than  the  other 
two  colours  will  be  deposited  first,  then  the  yellow, 
and,  lastly,  the  highly  soluble  blue.  So  that  ultimately 
we  get  three  layers  just  as  in  Szczepanik’s  process, 
the  only  difference  being  that  the  single  layer  resolves 
itself  ultimately  into  three  very  thin  layers,  whereas 
the  triple  layer  remains  as  a single  thick  layer.  The 
Uto-color  process  has  therefore  the  advantage  of  the 
three  layers  being  closer  together,  and  hence  there  is 
less  parallax.  Moreover,  the  Uto-color  method  can 
be  produced  much  more  cheaply,  since  there  is  only 
one  layer  to  coat  instead  of  three.  On  the  other 
hand,  with  Szczepanik’s  method  one  may  isolate 
each  layer  in  turn  and  so  prevent  the  three  dyes 
from  acting  chemically  on  each  other,  this  isolation 
allowing  of  a larger  range  of  colours.  Szczepanik  has 
now  overcome  the  difficulty  of  isolating  and  fixing  the 
intermediate  layer,  which  was  at  first  a serious  objection. 
He  does  this  by  spreading  on  first  a layer  of  red  stained 
gelatine,  then  a coating  of  varnish  which  has  been 
stained  yellow;  and,  lastly,  a blue  stained  layer  of 
gelatine. 

In  practice  both  methods  will  produce  equally  good 
prints.  The  Uto-color  process  is,  however,  the  one 
most  in  favour  at  the  present  time. 


UTO-COLOR  PRINTING  1 89 

Methods  of  Increasing  the  Sensitivity  of  the  Dyes. — 
The  dyes  alone  are  not  sufficient.  It  is  necessary 
to  increase  their  sensitivity  to  light.  This  is  effected 
by  means  of  either  an  ethereal  oil  such  as  Anethol,  or 
Thiosiamine,  which  is  a compound  of  Urea,  discovered 
by  Dr.  Smith,  the  inventor  of  the  Uto-color  process. 
It  is  interesting  to  note  that  Urea  itself,  or  one  of  its 
products,  has  been  used  by  platemakers  to  increase  the 
sensitivity  of  plates  for  many  years  past.  Peroxide  of 
Hydrogen  has  also  been  recommended,  but  as  it  rapidly 
decomposes  into  oxygen  and  water  it  is  of  no  value. 
The  special  qualifications  of  a good  sensitiser  are  that 
it  should  be  permanent  as  far  as  its  composition 
goes,  it  should  be  non-volatile,  and  easily  removable. 
There  is  still  a great  demand  for  a good  sensitiser, 
as  all  those  which  we  know  of  possess  certain  draw- 
backs. 

In  selecting  a support  for  the  gelatine  containing  the 
three  colours,  the  paper  should  be  perfectly  neutral 
and  indifferent,  i.e.  it  should  possess  no  chemical 
properties  which  can  act  on  the  gelatine,  and  it  must 
be  protected  by  means  of  an  insulating  material  from 
becoming  stained  by  the  dyes.  When  the  Uto  paper 
was  first  issued  it  was  found  to  be  stained  pink  by  the 
acid  red  dye  (Erythrosine)  which  leaked  out  of  the 
gelatine.  At  the  present  time  a basic  red  dye  is  used 
instead,  being  better  adapted  for  several  reasons. 

§ 95.  The  Nature  of  Dyes. — Dyes  are  of  three 
kinds,  basic,  acid,  and  neutral.  Basic  dyes  are  salts  of 
the  colour  bases,  which  latter  form  the  real  colouring 
principle.  It  may  be  just  as  well  to  explain  here  the 
meaning  of  a few  terms  which  are  constantly  used  in 


190  PHOTOGRAPHY  IN  COLOURS 

connection  with  this  subject,  viz.  Bases,  salts,  acids, 
and  colloids. 

A Base  is  a substance  which  combines  with  an  acid, 
and  neutralises  it  to  form  a salt.  It  is  usually  an 
oxide,  and  sometimes  a derivative  of  Ammonia,  and 
when  it  combines  with  an  acid,  water  is  formed. 
Bases  are  nearly  always  alkaline,  or  if  not,  neutral,  and 
have  the  opposite  character  to  acids.  Thus  a Base 
usually  turns  red  litmus  paper  blue,  whereas  an  acid 
turns  blue  litmus  paper  red.  When  it  unites  with  an 
acid  the  acid  is  neutralised,  and  a salt  is  formed.  For 
example,  Sodium  Carbonate  (Na2C03)  is  a base.  If 
Hydrochloric  acid  (HC1)  be  added  to  the  base,  a neutral 
salt  (common  salt,  or  NaCl)  is  formed,  which  is  com- 
posed of  the  metal  sodium  and  chlorine  gas,  while  water 
and  carbonic  acid  are  liberated.  A neutral  body  such 
as  common  salt  is  neither  acid  nor  alkaline,  and  there- 
fore has  no  effect  on  litmus  paper. 

A Salt , then,  is  a compound  formed  when  the 
Hydrogen  in  an  acid  is  partly,  or  entirely,  exchanged 
for  a metal. 

An  Acid  is  a substance  containing  Hydrogen  which 
reddens  litmus  paper ; and  when  it  unites  chemically 
with  a base,  the  Hydrogen  is  replaced  by  the  metal  of 
the  base  (if  one  is  present)  and  a salt  is  formed.  Thus 
KHO  and  K20  are  bases.  Sulphuric  acid  (H2S04)  is 
an  acid.  If  one  of  the  two  comes  in  contact  with  the 
acid,  either  one  or  two  atoms  of  Potash  are  capable  of 
exchanging  places  with  the  Hydrogen  to  produce  one 
or  other  of  the  salts,  Potassium  Sulphate  (K2S04)  or 
Hydro-potassium  Sulphate  (KHS04). 

A Colloid  is  a viscous  or  jelly-like  substance,  which 


UTO-COLOR  PRINTING 


I9I 

in  solution  will  not  pass  through  an  animal  membrane. 
Indiarubber,  Gelatine,  and  Collodion  are  examples  of 
colloids.  They  are  very  intractible  and  difficult  to 
analyse.  A colloid  is  necessary  to  hold  the  colour  in 
solution  in  a printing-out  paper. 

Acid  and  Basic  Colours . — These  are  nearly  all 
Analines  (Coal-tar  products),  and  comprise  the  vast 
majority  of  dyes  used  in  commerce  to-day.  The  real 
colouring  matter  of  such  a dye  lies  in  the  colour  base. 
If  such  base  is  combined  with  an  acid  it  forms  a basic 
dye  which  is  merely  a salt  of  a colour  base.  The  most 
important  dyes  used  in  colour  photography  are  the 
following 

Reds. — Ehodamine,  Saffranine. 

Blues . — Methyl  violet,  Diamine  blue,  Methylene 
blue. 

Yellows. — Chrysophenine,  Auromine,  Chrysoidine. 

Acid  Dyes. — These  form  by  far  the  largest  group 
of  dyes.  They  are  all  salts  of  the  colour  acids.  Just 
as  in  the  former  class  a colour  base  or  alkali  forms  the 
real  colouring  principle,  so  in  this  case  an  acid  colour 
body  does  the  same.  When  this  acid  is  combined  with 
a colourless  base,  e.g.  Soda,  Lime,  or  Ammonia  com- 
pounds, we  get  an  acid  salt.  And  as  in  the  former 
case  the  acid  has  no  effect  on  the  colour,  so  in  this 
case  the  addition  of  the  base  neutralises  the  acid,  and 
consequently  does  not  change  the  colour. 

Acid  dyes  are  of  course  acid,  and  they  dye  substances 
best  in  an  acid  bath.  The  chief  acid  dyes  used  in 
colour  photography  are  as  follows  : — 

Reds. — Echt  Eot,  Glycinred,  Krapplak,  Biebrich 
scarlet,  Erythrosine,  Irisine,  Isocyanine,  Eose 


192 


PHOTOGRAPHY  IN  COLOURS 


Bengal,  Eosine,  Methyl  red,  Nitrate  of  AEthyl- 
red,  iEthylred,  Chinolin  red. 

Yellows. — Phenosaffranine,  Rapid  filter  yellow 
(Hoechst),  Naphthol  yellow,  Normal  gelb,  Pina- 
chrome,  iEsculine,  Methyl  orange,  Anrantia, 
Picric  acid,  H.  Naphthol  orange,  Eosine  yellow. 
Blues.  — Cyanine,  Fast  blue,  Methylene  blue, 
Normal  blue,  Milori  blue.  Peacock  blue. 

Greens. — East  Green,  Acid  green. 

Mineral  and  Vegetable  Colours.  — These  form  a 
class  by  themselves,  and  are  largely  used  for  printing- 
inks  in  colour  photography.  Such  are,  for  example  : — 
Chrome  yellow,  Cadmium  yellow,  Zinc  yellow,  Prussian 
blue,  Madder  lake,  and  its  artificial  derivative  Ali- 
zarin. They  are  all  mordants  (that  is,  self-fixing),  and 
are  very  permanent. 

All  the  dyes  required  in  photography  may  be  obtained 
from  one  or  other  of  the  following  firms  : — 

1.  Actien  Ges.  fur  Analinfabrikation, 

Hoechst  factory,  Frankfort-a-M. 

2.  Bayer  Elberfeld,  Germany. 

3.  Berger  u.  Wirth,  Benthstrasse,  Berlin,  for  Normal 
Blue  and  Normal  Red. 

4.  Bohringer  Mannheim  (for  JEthylred). 

Grubler  und  Holborn,  Chemiker,  63,  Baierische 
Strasse,  Leipsig,  or  their  agent,  Charles  Baker, 
244,  High  Holborn,  London,  W.C. 

East  in  Ehinger  (for  inks),  A.  and  E.  Lumiere,  Lyons. 

Dr.  G.  Mundie,  Mik.  Chem.  Institut,  Gottingen, 
Hanover. 

§ 96.  Uto=color  Paper. — In  the  year  1906,  Dr. 
J.  H.  Smith,  of  Zurich,  produced  a paper  covered  with 


UTO-COLOR  PRINTING 


x93 


a gelatine  layer  impregnated  with  the  three  primary 
colours.  He  called  it  Uto  paper,  the  name  being 
taken  from  a range  of  mountains  near  Zurich.  This 
paper  was  at  first  of  a greyish-green  colour,  but  later 
efforts  produced  a black  paper,  which,  as  we  have 
already  pointed  out,  is  the  correct  and  in  fact  the  only 
possible  colour  which  the  paper  can  have  to  give  the 
effect  of  dark  shadows.  Obviously  Black  cannot  be 
formed  by  any  conceivable  bleaching  action,  and  if  any 
black  is  to  appear  at  all  in  the  finished  picture,  it  must 
be  supplied  by  the  unaltered  paper.  This  paper  proved 
very  disappointing  and  unsatisfactory  owing  to  the 
irregular  way  in  which  the  colours  printed-out,  and  to 
the  great  want  of  sensitiveness  of  the  colours  to  light. 
Besides,  in  order  to  sensitise  it  before  printing  it  was 
necessary  to  add  Peroxide  of  Hydrogen.  Subsequently, 
Dr.  Smith  discovered  that  Anethol  was  not  only  a better 
sensitiser,  but  could  be  incorporated  in  the  colour  layer, 
thus  rendering  the  process  of  after-sensitising  un- 
necessary. 

Uto  Rapid  Paper.  This  paper  is  now  manfactured 
and  sold  by  the  Society  Anonyme  Uto-colour,  Rue  de 
la  Pointe,  La  Garenne-Golombes,  Paris.  It  was  intro- 
duced in  October,  1911,  and  perfected  a year  later,  and 
is  a great  improvement  over  the  old  paper.  It  consists 
of  a paper  covered  with  the  following  layers  : — 

1.  The  white  paper  support. 

2.  A white  Baryta  layer,  which  gives  the  pure 
whites  in  the  finished  picture  after  the  colour  layers 
are  entirely  bleached-out.  The  reader  must  re- 
member that  in  the  print  we  are  dealing  with  colour 
formation  by  subtraction  (subtractive  process,  see 

o 


194 


PHOTOGRAPHY  IN  COLOURS 


p.  68),  whereas  in  the  formation  of  the  positive  the 
colours  are  produced  by  addition  (additive  process  or 
colour  synthesis).  In  the  paper  print  the  whites  are 
produced  by  more  or  less  complete  removal  of  all 
three  colours  in  the  gelatine  layer.  The  object  of  the 
Baryta  layer  is  to  afford  a purer  white  than  the  paper 
alone  would  give. 

3.  The  colourless  insulating  layer.  This  layer  is 
necessary  to  prevent  the  colours  from  soaking  through 
and  staining  the  Baryta  layer  and  the  paper  beneath. 

4.  The  Gelatine  Emulsion  Layer.  This  holds  the 
mixed  red,  yellow,  and  blue  dyes,  which  are  adjusted 
to  form  as  dense  a black  as  possible,  and  give  rise  to 
the  blacks  and  dark  shadows  in  the  finished  picture. 
The  black  has  a slightly  greenish  tinge,  but  this  is 
hardly  noticeable.  The  gelatine  emulsion  is  coated  by 
machinery,  and  holds  the  three  dyes  and  the  sensitisers 
Thiosinamine  and  Anethol,  and  other  compounds.  A 
little  glycerine  is  added  to  prevent  the  layer  from 
becoming  too  dry. 

Under  a suitable  positive,  the  colours  print  out 
fairly  evenly,  and  give  quite  a good  effect. 

According  to  the  makers,  the  paper  keeps  well  in  its 
original  wrapper  for  about  a year,  if  it  is  kept  in  a 
cool  dry  place.  Each  sheet  is  packed  with  the  sensitive 
surface  protected  by  a piece  of  brown  tissue  paper. 
This,  as  the  author  has  pointed  out,  is  a mistake,  as  in 
hot  climates  the  tissue  paper  will  stick  so  firmly  as  to 
render  it  impossible  to  remove  it.  He  has  suggested  to 
the  company  that  the  paper  should  either  be  packed  in 
air-tight  and  light-tight  grooved  boxes,  so  that  the 
black  surface  should  not  touch  anything,  or  else  the 


UTO-COLOR  PRINTING 


!95 


sheets  should  be  in  contact  with  celluloid  films  which 
would  not  stick.  If  the  sheets  become  too  dry,  the 
emulsion  films  become  brittle  and  crack;  if  they  are 
kept  in  a damp  place,  they  stick  to  the  positive,  and 
print  out  too  green. 

§ 97.  Practical  Details  of  Printing  on  Rapid 
Uto  Paper. 

1.  Subjects  for  Printing. — Any  colour  screen-plate 
will  do,  and  any  subjects  may  be  copied,  but  to  get  the 
best  results,  the  following  points  must  be  attended  to  : — 

(a)  The  colour  transparency  or  positive  should 
possess  strong  and  brilliant  colours  since  the  pictures 
always  lose  a good  deal  of  the  original  brilliancy  and 
appear  flat.  This  is  due  to  the  fact  that  the  paper 
pictures  are  seen  by  reflected  light,  and  not  by  trans- 
mitted light  as  the  originals  are. 

( b ) The  positives  should  not  possess  great  contrasts 
of  light  and  shade,  because  to  obtain  these  the  colours 
must  be  entirely  bleached  in  some  parts  and  remain 
unaffected  in  others,  and  that  is  a result  which  is  very 
difficult  to  obtain,  since  the  light  parts  print  out  too 
quickly  compared  with  the  dark  parts,  which  we  know 
is  the  case  in  ordinary  black  and  white  photography. 

(c)  The  illumination  must  be  evenly  distributed  over 
the  entire  plate.  The  best  subjects  for  printing  are, 
therefore,  positives  which  have  rich  saturated  colours, 
without  possessing  great  contrasts  of  light  and  shade, 
i.e.  subjects  which  are  not  too  hard. 

2.  Preparing  the  Subjects  for  Printing. — The  positive 
must  first  be  coated  with  a special  varnish  supplied  by 
the  Uto-color  Company,  since  the  ordinary  varnishes 
have  too  low  a melting  point.  If  the  Author’s  alum 


196  PHOTOGRAPHY  IN  COLOURS 

trough  dish  be  used  no  varnish  is  required,  since  the 
alum  trough  absorbs  all  the  heat.  The  plate  must  first 
be  gently  and  evenly  warmed  (but  not  heated,  or  the 
film  will  be  injured),  and  the  plate  is  then  held  either 
between  the  fingers  at  the  sides,  or  on  a pneumatic 
rubber  glass  holder,  and  the  varnish  is  then  liberally 
poured  over  the  middle  of  the  plate,  and  by  gently 
rocking  the  plate,  is  made  to  distribute  itself  up  to  the 
four  corners  alternately,  and  the  remainder  is  then 
poured  back  into  the  bottle  by  tilting  up  the  opposite 
end.  A little  practice  will  enable  the  beginner  to  pour 
it  on  evenly.  The  great  point  is  not  to  have  the 
varnish  too  concentrated,  otherwise  it  becomes  very 
difficult  to  apply  evenly.  The  beginner  should  practise 
it  on  a spoilt  negative  a few  times  first,  as  he  is  apt  to 
spoil  the  picture  when  he  tries  to  dissolve  it  off  in  order 
to  get  rid  of  the  lines  and  ridges.  The  plate  should  be 
gently  warmed  again  after  varnishing  until  it  is  no 
longer  sticky,  but  has  become  quite  dry. 

3.  Printing  the  Picture. — This  is  conducted  exactly 
in  the  same  way  as  with  an  ordinary  printing-out 
paper.  The  paper  is  taken  out  of  the  packet  in  one’s 
shadow  at  the  end  of  the  room  away  from  the  window, 
the  black  side  of  the  paper  being  in  contact  with  the 
film  of  the  positive  in  the  printing  frame.  The  Author 
invariably  fits  a plate  of  clear  glass  inside  the  frame 
against  which  the  glass  side  of  the  negative  rests. 
This  is  advisable  for  many  reasons,  and  is  indispensable 
if  the  negative  (or  positive)  is  smaller  than  the  frame. 
When  all  is  adjusted,  the  frame  is  placed  in  the  open 
air  and,  if  possible,  in  direct  sunshine,  so  as  to  reduce 
the  time  of  printing. 


UTO-COLOR  PRINTING 


197 


The  Author  has  already  recommended  the  use 
of  an  alum  trough.  This  consists  of  either  an  all- 
glass trough,  or,  what  is  better,  a sheet  of  ordinary 
clear  glass  fixed  with  putty  into  the  bottom  of  a wooden 
frame  about  1J  ins.  high.  This  is  then  filled  half- 
way up  with  a 3 per  cent,  solution  of  common 
alum  in  tap  water  (that  is,  about  15  grains  to  the 
ounce  of  water).  The  trough  should  be  a little  larger 
than  the  frame  so  as  to  prevent  any  shadows  being 
on  the  picture.  The  object  of  the  alum  trough  is  to 
absorb  all  the  heat  rays,  which  latter  would  otherwise 
tend  to  injure  the  positive  and  soften  the  gelatine  layer 
of  the  paper  Any  scratches  or  marks  or  unevenness 
in  the  plate  at  the  bottom  of  the  trough  will  not  affect 
the  print,  since  the  glass  is  too  far  away  from  it. 

The  time  of  exposure  varies  from  an  hour  to  three 
hours  in  the  full  sunlight  in  summer,  according  to  the 
nature  and  density  of  the  positive.  In  diffused  day- 
light the  time  will,  of  course,  be  considerably  increased. 
From  time  to  time  a corner  of  the  print  should  be 
examined  to  see  how  the  printing  is  going  on. 

Sometimes  the  prints  will  be  found  to  stick  to  the 
positives.  This  always  occurs  if  either  the  varnish  or 
the  black  colour  layer  is  not  perfectly  dry,  but  it  may 
occur  even  if  they  are  dry.  This  will  happen  if  the 
gelatine  layer  of  the  paper  or  the  varnish  of  the  positive 
gets  too  hot  during  the  printing  in  the  sun.  We  can 
avoid  this  in  several  ways.  We  may  largely  prevent  it 
by  using  the  alum  trough  above  mentioned.  Or  we  may 
put  one  or  two  drops  of  olive  oil  on  the  centre  of  the 
black  layer  of  the  paper,  and  gently  rub  it  evenly  over 
the  surface  with  the  finger,  or  a smooth  soft  pad,  being 


198  PHOTOGRAPHY  IN  COLOURS 

careful  not  to  put  too  much  on  so  as  to  leave  rings  or 
lines  of  oil  behind.  These  must  be  mopped  up,  or 
they  will  leave  marks  in  the  printing.  Lastly,  we  may 
place  a very  thin  layer  of  celluloid  between  the  two 
surfaces,  a piece  of  a Kodak  roll-film  carefully  cleaned 
will  do  perfectly. 

The  Uto  people  claim  that  more  than  a hundred 
prints  can  be  made  from  a single  diapositive,  but  they 
do  not  say  what  effect  such  prolonged  exposure  to  the 
sun’s  rays  will  have  on  the  originals.  For  my  part  I 
would  not  risk  more  than  two  or  three  prints  off  it. 
Should  a greater  number  be  required,  it  would  be 
much  safer  to  take  a copy  by  direct  contact,  or  through 
the  camera,  and  then  print  off  the  copy. 

§ 98.  Methods  of  Improving  the  Print. — It  is  found 
to  be  very  difficult  to  keep  the  blacks  pure,  while  at  the 
same  time  bleaching  out  the  high  lights  so  as  to  get 
good  pure  whites.  Worel,  Miethe,  and  others  suggest 
overcoming  the  difficulty  by  making  a negative  copy 
on  an  ordinary  gelatine  plate,  and  then  taking  a posi- 
tive from  that.  This  black  and  white  positive  is  then 
substituted  for  the  colour  positive  and  a preliminary 
print  is  made  on  the  Uto  paper  so  as  to  bleach  out  the 
high  lights.  The  colour  positive  can  then  be  substi- 
tuted for  the  black  and  white  positive  (being  careful  to 
secure  exact  register),  and  the  colours  printed  out  in 
the  usual  way. 

Printing-filters  of  yellowish-green  of  different  shades 
(marked  G and  MG,  MG  being  twice  as  dense  as  G), 
can  be  had  from  the  Uto-color  firm.  Either  one  of 
these  may  be  placed  over  the  positive  so  that  all  the 
light  will  filter  through  it.  These  filters  are  useful  to 


UTO-COLOR  PRINTING 


199 


check  unequal  printing.  Thus,  if  the  blue  and  yellow 
dyes  bleach  out  quicker  than  the  red  (which  is  usually 
the  case)  a green  filter  will  allow  most  of  the  blue  and 
yellow  rays  to  pass  through,  and  very  few  red  rays. 
According  to  the  bleach-out  law,  since  the  blue  and 
yellow  rays  pass  through  the  filter,  they  will  to  a great 
extent  bleach  out  the  opposing  colour  red,  but  will 
only  slightly  affect  the  other  two  colours.  Hence  by 
using  the  filter  for  a certain  time,  we  can  regulate  the 
effect  on  the  three  colours,  so  as  to  get  an  even  balance 
between  them.  In  the  same  way,  if  the  green  colour 
tends  to  predominate  we  can  adjust  matters  by  using 
a light  red  filter.  According  to  the  Uto-color  manu- 
facturers, if  the  paper  is  slightly  damp,  greens  will 
predominate,  or  in  other  words  the  blue  and  yellow 
colours  will  bleach  out  faster  than  the  red. 

§ 99.  Fixing  the  Uto=color  Prints. — The  prints 
must  now  be  fixed.  This  is  effected,  1st,  by  removing 
the  sensitive  products  resulting  from  the  bleaching  out 
of  the  dyes,  and  2nd,  by  making  the  dyes  stable  to  light. 
Unfortunately  up  to  the  present  neither  of  these 
essentials  can  be  perfectly  fulfilled.  Still  the  prints 
can  be  fixed  quite  sufficiently  to  render  the  colours 
permanent  if  kept  in  an  album.  If  mounted  and  hung 
on  the  walls  they  must  be  placed  where  no  direct  light 
from  the  window  can  reach  them.  If  the  print  has 
been  oiled,  the  oil  must  first  be  removed  with  a soft 
pad  soaked  in  benzine.  Then  the  picture  is  placed  for 
half  an  hour  in  a bath  containing  35  grams  of  Tannin 
to  100  c.c.  of  methylated  spirits  (15  grains  to  the  ounce). 
About  3^  ounces  will  suffice  for  4 quarter-plate  prints. 
The  tannin  solution  dissolves  out  the  sensitisers  and 


200 


PHOTOGRAPHY  IN  COLOURS 


bleaching  products,  and  in  addition  hardens  the  film. 
Next  wash  the  print  for  three  minutes  under  the  tap, 
and  then  place  it  in  the  fixing  bath.  This  bath  con- 
sists of  a liquid  sold  by  the  Uto-color  Co.  in  a concen- 
trated form.  It  is  a green  strongly  smelling  liquid. 
One  ounce  of  this  is  to  be  diluted  with  3 ozs.  of  water. 
The  prints  should  be  left  in  for  about  three  minutes. 
They  are  then  rinsed  for  about  a minute  under  the  tap, 
and  dried  as  quickly  as  possible.  This  is  best  done  by 
squeegeeing  the  print  film  downwards  on  to  a sheet  of 
glass,  or  japanned  iron  if  a glossy  surface  be  desired, 
or  ground  glass  for  a matt  one.  The  back  of  the  print 
is  then  dried  with  blotting  paper,  and  the  print  left  to 
dry.  The  glass  on  which  the  print  is  squeegeed  must 
first  be  thoroughly  cleansed,  and  then  wiped  over  with 
French  chalk,  or  a little  wax  dissolved  in  benzole  in 
order  to  prevent  the  film  sticking. 

§ 100.  Uto= color  Stripping- paper. — When  a 

photograph  is  taken  on  an  ordinary  plate  the  picture 
is  reversed,  i.e.  the  right  side  of  the  object  appears  on 
the  left,  and  the  left  on  the  right  side  of  the  negative, 
but  when  the  print  is  made  the  position  is  reversed 
again,  so  that  the  print  corresponds  to  the  object.  If 
a colour  positive  is  taken,  the  position  of  the  object  is 
correct,  because  the  plate  is  put  into  the  camera  with 
the  glass  side,  and  not  the  film  side  towards  the  lens ; 
and  so  when  a print  is  made,  whether  coloured  or 
plain,  the  position  is  reversed.  The  picture  will  only 
correspond  to  the  original  when  it  is  held  up  to  the 
light  and  looked  at  from  behind. 

One  may  obtain  a correctly  placed  picture  in  three 
ways. 


UTO- COLOR  PRINTING 


201 


1st.  We  may  use  a reversing  prism  when  taking  the 
positive,  and  the  print  made  from  this  will  be  correct. 
But  reversing  prisms  are  very  expensive,  and  few 
amateurs  possess  one. 

2nd.  We  may  employ  a colour-film  fixed  on  a very 
thin  celluloid  sheet,  and  print  through  the  back.  Such 
films  can  be  had  from  the  Neue  Photographische 
Gesellschaft  (New  Photographic  Company),  Steglitz, 
Berlin. 

3rd.  We  may  use  a bleach-out  paper  which  can  be 
stripped  like  carbon  tissue.  This  is  now  supplied  by 
the  Uto-color  Co.,  and  is  likely  to  supersede  the 
ordinary  rapid  Uto-color  paper. 

The  stripping-paper  is  used  exactly  in  the  same  way 
as  the  Uto  paper  just  described.  It  may  be  oiled  just 
before  printing.  Then  the  film  is  lifted  up  at  one 
corner  with  a penknife,  and  stripped  off  with  the 
fingers.  It  is  then  carefully  laid  on  the  film  side  of 
the  positive,  and  a piece  of  black  paper  is  laid  on  the 
top  of  it.  The  cover  of  the  frame  is  placed  on  it  and 
fastened  down,  and  the  positive  exposed  to  the  light. 

In  order  to  watch  the  progress  of  the  printing,  one 
half  of  the  frame-cover  is  lifted  up,  and  a sheet  of 
white  paper  is  inserted  a little  way  between  the  posi- 
tive and  the  film,  since  it  is  impossible  to  see  the 
picture  on  a black  support. 

After  printing,  the  film  is  placed  on  a glass  plate 
and  the  oil  removed  with  a pad  soaked  in  benzine.  It 
is  then  laid  face  upwards  on  a piece  of  stout  Baryta 
paper  which  should  be  slightly  larger  than  the  film.  A 
sheet  of  paper  or  cardboard  is  now  selected  on  which 
to  mount  the  film,  and  the  latter  with  its  Baryta 


202 


PHOTOGRAPHY  IN  COLOURS 


support  is  squeegeed  face  downwards  on  to  the  mount. 
As  soon  as  the  gelatine  has  set,  the  Baryta  support  is 
pulled  off,  leaving  the  film  (now  right  side  up)  on  the 
paper  or  cardboard  mount.  It  is  then  left  to  dry  in  a 
dark  place. 

§ 101.  Uto  Lantern  Slides. — The  stripped  Uto-color 
film  may  be  printed  and  mounted  on  a glass  plate  in- 
stead of  on  paper,  and  thus  can  be  used  as  a grainless 
lantern  slide.  The  glass  plate  must  be  dipped  into  a 
3 per  cent,  solution  of  gelatine,  just  as  was  done  in  the 
case  of  the  mount,  and  the  film  squeegeed  on,  while  the 
gelatine  is  hot.  It  is  recommended  to  cover  the  film 
with  a thin  sheet  of  rubber  to  prevent  the  film  being 
damaged  while  it  is  being  squeegeed.  The  glass  trans- 
parency is  then  allowed  to  dry  in  a dark  place,  and 
afterwards  fixed  in  the  usual  way,  except  that  a 3 per 
cent,  solution  of  Chrome  Alum  is  to  be  preferred  to 
the  tannin  bath.  A cover  glass  is  finally  added  and 
bound  round  with  binding  strips,  or,  what  is  much 
better,  with  a long  strip  of  adhesive  surgical  plaster 
ygths  of  an  inch  (11  mm.)  in  diameter.  Uto  lantern 
slides  possess  much  more  brilliancy  than  Uto  papers 
since  they  are  seen  by  transmitted  light,  and  moreover 
they  do  not  require  to  be  printed  quite  so  deeply. 
Whether  their  colours  will  stand  the  prolonged  action 
of  the  luminant  remains  to  be  proved. 


CHAPTER  XII 


KINEMATOGRAPHY  BY  MEANS  OF  COLOURED  LIGHTS 

§ 102.  Projection  of  [Cinematograph  Pictures  in 
Colour.  — The  extraordinary  popularity  of  screen 
pictures  of  moving  objects  has  led  to  innumerable 
attempts  to  still  further  the  illusion  by  representing 
the  scenes  in  colour.  This  has  been  frequently  done 
by  staining  the  film,  and  in  this  way  effects  in  which 
browns  or  greens  are  predominant,  or  moonlight 
scenes,  can  be  obtained ; but  although  such  effects 
are  pleasing  at  first,  the  eye  soon  gets  tired  of  the 
unreality.  Colouring  the  films  by  hand  is  possible, 
but  tedious  beyond  measure.  When  one  considers 
that  many  films  measure  1000  feet  and  contain  over 
16,000  separate  pictures,  the  time  and  expense  required 
to  colour  such  a film  are  very  great.  In  spite  of 
the  difficulties,  hand-coloured  films  are  still  produced 
to  a considerable  extent.  The  colours  are  never  brilliant 
and  as  far  as  I can  ascertain  are  limited  to  pink, 
bluish-green,  yellow  and  brown.  Notwithstanding 
this  the  films  are  quite  a success,  and  in  the  writer’s 
opinion  are  far  more  pleasing  and  agreeable  to  the  eye 
than  the  kinemacolor  films. 

§103.  The  Urban=Smith  “Kinemacolor”  Pro- 
cess. — Mr.  Charles  Urban  and  his  collaborator,  Mr.  G. 
Albert  Smith,  after  a prolonged  series  of  experiments, 


204  PHOTOGRAPHY  IN  COLOURS 


have  at  length  succeeded  in  exhibiting  kinematograph 
pictures  in  colour,  which  for  brilliance  of  colour  effect  are 
unequalled  by  any  other  method.  Although  only  two 
colour- filters  are  used  in  projection,  a red  and  a green, 
these  brilliant  colours  and  their  orange  and  yellow 
combinations,  as  well  as  browns,  greys,  eau-de-nil , and 
even  blues  and  indigoes,  are  in  evidence.  The  green 
filter  used  is  one  which  transmits  a considerable 
amount  of  blue  light,  and  therefore  the  resultant 
picture  gives  not  only  the  effect  of  blue  sky  and  water, 
but  a very  considerable  range  of  combination  of  all  the 
colours,  as  well  as  white  and  black. 

The  principle  of  the  Kinematograph  depends  on 
what  is  called  “persistence  of  vision”  and  the  con- 
tinued perception  of  the  changing  object.  When  light 
is  reflected  from  a moving  object  it  forms  an  image  at 
the  back  of  the  eye,  and  produces  a nerve  current  which 
passes  along  every  one  of  the  fibres  which  receive  the 
image  and  collectively  carry  the  impression  along  the 
optic  nerve  to  the  brain.  This  sensation  is  not  instan- 
taneous, but  is  divided  up  into  four  periods : 1st,  a 
latent  period  which  is  almost  instantaneous,  and 
during  which  nothing  appears  to  happen  ; 2nd,  a 
very  short  period — probably  less  than  of  a second 
— during  which  the  sensation  reaches  the  maximum ; 
3rd,  a much  longer  period,  N to  ^ of  a second 
(the  time  varying  directly  with  the  intensity  of 
the  illumination),  during  which  the  sensation  slowly 
diminishes  ; and  4th,  a short  period  of  decline, 
during  which  the  effect  dies  away.  In  the  case  of 
a moving  object  on  which  attention  is  directed,  the 
fourth  period  remains  unnoticed,  because  a new  image 


/CINEMA  TOGRA PHY  IN  COLOUR  205 


takes  the  place  of  the  old  one  before  that  period 
arrives.  The  whole  of  kinematography  depends  on 
this  third  period,  by  which  the  first  impression,  A 
(Fig.  26),  lingers  until  replaced  by  the  second  one,  B, 
and  the  second  one  is  again  replaced  by  a third  one,  G, 
and  so  on. 

We  have  suggested  this  in  the  diagram,  in  which  the 
height  of  the  curve  represents  the  intensity  of  the 
light  stimulus,  and  the  width  or  base  the  time. 


Fig.  26. — Curve  representing  the  four  periods  of  a visual  sensa- 
tion. The  first  sensation  is  represented  by  a thick  line,  the 
two  succeeding  ones  by  dotted  lines.  1.  Short  latent  period ; 
2.  Period  of  increase  to  maximum ; 3.  Long  period  of  per- 
sistence of  sensation  with  gradual  decline ; 4.  Period  of 
rapid  fall  and  obliteration  of  the  sensation.  In  reality  the 
sensations  overlap  very  much  more  than  here  shown,  but 
they  are  separated  in  the  diagram  for  the  sake  of  clearness. 


This  explains  why,  when  a lighted  stick  is  whirled 
round,  it  forms  an  unbroken  circle  of  fire,  and  why  a 
stream  of  water  allowed  to  drop  from  a pipe  appears 
to  form  a continual  stream,  and  not  a series  of  droplets, 
as  is  really  the  case  ; and  just  as  the  first  impression  of 
a moving  object  melts  into  the  next  one,  so  a series  of 
colours  pass  before  the  eye,  as  in  the  familiar  colour-top 


206 


PHOTOGRAPHY  IN  COLOURS 


which  carries  a card  divided  into  sections  painted 
blue,  green,  and  red.  If  the  top  be  spun  rapidly,  each 
colour  fuses  into  the  next,  and  a combined  sensation 
of  white  appears  to  the  eye.  It  is  this  last  prin- 
ciple that  Mr.  Urban  and  Mr.  Smith  have  so  cleverly 
made  use  of  in  their  “ Kinemacolor  ” apparatus. 

In  working  out  his  method  of  kinematograph  pictures 
in  colours,  Mr.  Smith  based  his  first  experiments  on  an 
instrument  somewhat  similar  to  the  Ives’  Kromskop, 
and  also  on  the  same  inventor’s  triple  projecting 
lantern.  The  principle  of  the  analysis  of  the  colours 
in  the  object  photographed,  and  the  subsequent  build- 
ing up  of  the  colour-records  to  produce  a coloured 
result,  are  similar  in  both  cases.  In  his  earliest  experi- 
ments, he  made  use  of  strip-film  negatives  taken 
alternately  through  red,  green,  and  blue  filters.  When 
he  had  made  a positive  film  from  this  negative  film, 
and  proceeded  to  project  his  pictures  on  to  the  screen 
by  red,  green,  and  blue  light  respectively,  the  results 
were  almost  colourless,  on  account  of  the  excessive 
actinic  action  of  the  blue  light  which  had  produced  the 
blue  negative  record;  and  the  correspondingly  over- 
powering effect  of  the  blue  light  which  reached  the 
screen  through  the  blue  filter.  This  obliterated  both 
the  other  two  images.  In  other  words,  the  exposure 
necessary  to  get  satisfactory  red  and  green  records 
was  utterly  out  of  scale  with  that  required  for  the 
blue  record. 

Another  serious  objection  to  the  use  of  red,  green, 
and  blue  was  that  the  normal  speed  of  the  kinemato- 
graph film,  which  is  one  foot  per  second  (each  foot 
carrying  sixteen  exposures),  required  to  be  increased 


KINEMATOGRAPHY  IN  COLOUR  20 7 


to  three  feet  per  second  (forty-eight  exposures  in  the 
same  time).  Such  an  increase  of  speed  would,  of 
course,  involve  prohibitive  expense  and  complicated 
and  expensive  mechanical  devices  for  the  manipulation 
of  the  films  at  this  high  speed. 

Further  experiments  with  the  Ives’  Kromskop  and 
the  comparison  of  the  appearance  of  the  coloured 
image  when  viewed  in  daylight  (illuminated  with 
white  sky)  as  compared  with  artificial  illumination, 
led  Mr.  Smith  to  the  following  discovery.  If  the 
blue  light  in  the  former  case  were  cut  off,  the  appear- 
ance of  the  coloured  image  was  utterly  spoilt.  In  the 
latter  case,  however,  the  blue  light  could  be  dispensed 
with  altogether  without  seriously  altering  the  effect  of 
the  coloured  image.  He  attributes  these  phenomena 
to  the  fact  that  most  artificial  lights  are  very  deficient 
in  blue — a fact  well  known  to  every  photographer. 
Our  eyes  are  to  some  extent  accustomed  to  this  excess 
of  red  and  green  rays  ( i.e . yellow  rays)  and  deficiency 
of  blue  ones. 

As  a result  of  the  above,  Mr.  Smith  has  perfected  the 
“ Kinemacolor  ” apparatus  to  use  red  and  green  filters 
only,  the  want  of  blue  being  met  by  using  a green  filter 
which  passes  a considerable  amount  of  blue  light. 

Herr  Kasimir  Proszynski  in  a Paper  read  before 
the  Eoyal  Photographic  Society  (see  Journal  R.P. 
Society  for  March,  1913)  considers  that  the  continuity 
of  kinematograph  vision  is  a purely  psychological 
illusion,  and  is  not  dependent  on  the  persistence  of 
vision  hitherto  believed  to  be  the  cause.  He  employs 
a shutter  consisting  of  three  similar  wings  separated 
by  three  intervals  of  precisely  the  same  shape,  for 


208  photography  in  colours 


according  to  him  three  wings  are  necessary.  By 
revolving  this  shutter  15  times  a second,  we  obtain 
45  alternations  of  light  and  dark,  and  15  separate 
pictures,  which  is  just  sufficient  to  eliminate  all  flicker- 
ing. He  states  that  intervals  of  ^th  to  ^th  of  a second 
comprise  the  limit  of  our  perception,  and  that  this 
interval  is  necessary  to  avoid  flickering.  Although  the 
writer  has  studied  Herr  Proszynski’s  paper  carefully, 
he  cannot  see  anything  in  it  which  refutes  the  usually 
accepted  theory,  nor  does  he  see  wherein  his  modifica- 
tion of  the  shutter  is  superior  to  the  ordinary  form 
in  use. 

§ 104.  The  “ Kinemacolor”  Camera  is  similar  to 
that  used  in  ordinary  black  and  white  kinematography, 
except  that  it  is  built  to  run  at  twice  the  speed,  viz.  two 
feet,  or  thirty-two  exposures,  per  second.  The  shutter 
used  is,  of  course,  a rotary  one,  and  so  geared  to  the 
handle  by  which  the  film  is  moved  that  the  light  only 
reaches  the  film  whilst  it  is  at  rest.  The  essential 
difference  between  the  ordinary  kinematograph  camera 
and  the  “Kinemacolor”  camera  is  that  the  latter  has 
a rotating  colour-filter  which  is  placed  between  the 
lens  and  the  shutter.  This  filter  consists  of  an 
aluminium  skeleton  wheel  having  one  segment  filled 
in  with  red-dyed  gelatine,  and  a similar  one  filled  in 
with  green-dyed  gelatine,  and  is  so  geared  that  the 
exposures  are  made  through  the  red  gelatine  and  the 
green  gelatine  filters  alternately.  The  film,  when  thus 
exposed,  is  developed  and  from  it  a positive  film  is 
made  by  contact  in  the  ordinary  way. 

If  the  negative  “ Kinemacolor  ” film  be  examined  it 
will  be  found  to  consist  of  images  in  pairs,  which 


Plate  XI. 


Through 
green  filter. 


Through 
red  filter. 


Through 
green  filter. 


Through 
red  filter. 


Kinemacolor  negative  film.  Kinemacolor  positive  film. 


To  fact  p.  208. 


/CINEMATOGRAPHY  IN  COLOUR  20g 


differ  from  each  other  inasmuch  as  they  are  records 
of  the  red  and  the  green  in  the  object  photographed. 

We  have  selected  for  our  illustration  (Plate  XI.)  four 
successive  pictures  of  a plant,  with  bright  red  flowers 
and  green  leaves,  standing  in  a brown  flower-pot.  The 
first  exposure  is  made  through  the  green  filter.  As 
nearly  the  whole  of  the  red  and  much  of  the  blue  are 
absorbed  by  the  filter,  the  red  flowers  in  the  negative 
will  show  hardly  any  silver  deposit,  while  the  green 
leaves  will  be  quite  dark,  and  the  brown  flower-pot 
will  have  an  intermediate  hue.  On  making  a positive, 
everything  will  be  reversed.  The  red  flowers  will  show 
a dense  deposit,  the  green  leaves  hardly  any,  and  the 
brown  flower-pot  an  intermediate  one. 

The  next  exposure,  taken  through  the  red  filter, 
will  exhibit  exactly  the  reverse.  If  you  look  at  the 
positive  (Plate  XI.)  you  will  notice  the  red  flowers  show 
hardly  any  deposit,  and  the  leaves  are  quite  dark,  while 
the  brown  pot,  since  it  is  merely  a mixture  of  red, 
green  and  a trace  of  blue  (all  degraded  with  black), 
has  the  same  density  in  both  positives. 

§ 105.  In  the  “ Kinemacolor  ” Projector  the  rotat- 
ing colour-filter  is  placed  between  the  condenser  and  the 
gate  or  fire-proof  shield.  When  the  two  pictures  are 
rapidly  projected  one  after  the  other  on  to  the  screen 
the  combined  effect  gives  rise  to  red  flowers  and  green 
leaves  in  a brown  flower-pot — as  it  should  be.  Had 
the  positive  film  been  placed  in  the  wrong  order,  i.e. 
the  “ green  ” picture  opposite  the  red  light,  the  colours 
would  have  been  the  complementary  ones,  all  through 
the  film.  So  in  order  to  prevent  mistakes,  a white 
dot  is  placed  on  the  film  opposite  the  “ green  ” picture 

p 


210 


PHOTOGRAPHY  IN  COLOURS 


in  the  black  and  white  image  which  has  to  be  projected 
by  green  light.  If  on  rotating  the  handle  the  colours  are 
seen  to  be  complementary,  it  may  be  instantly  remedied 
by  shifting  the  mask  one  whole  picture  width  either  up 
or  down,  and  at  the  same  time  recentring  the  arc  or 
limelight.  Furthermore,  whenever  a title  appears  on 
the  film,  it  should  be  so  threaded  as  to  show  red 
letters  on  the  screen.  If  this  is  done  all  the  colours 
will  come  in  their  right  order.  It  will  thus  be  seen 
that  with  a little  care  both  the  taking  and  exhibiting  of 
“ Kinemacolor  ” pictures  are  very  little  more  difficult  or 
troublesome  than  ordinary  black  and  white  pictures, 
while  there  can  be  no  doubt  that  with  films  correctly 
exposed  and  developed  and  in  perfect  register,  the 
pictures  in  colour  are  often  more  pleasing  to  the  eye 
than  the  ordinary  black  and  white. 

The  chief  defects  of  kinemacolor  pictures  are : 1st. 
If  the  objects  move  rapidly,  fringes  of  red  and  green 
are  seen  bordering  the  objects.  Thus,  if  a person 
suddenly  raises  his  bared  arm,  it  will  appear  at  one 
moment  red,  at  another  moment  pink,  and  again 
greenish ; or  else  the  arm  will  have  a fringe  of  red  or 
green.  2nd.  The  range  of  tints  and  hues  is  very 
limited.  This  is  inevitable,  seeing  that  all  the  effects 
are  produced  by  means  of  the  rotation  of  two  colours — 
an  intense  red,  and  an  intense  green  disc.  3rd.  The 
colours  are  painfully  intense  and  vivid.  There  is  an 
entire  absence  of  greys  and  neutral  tints,  which  are 
always  present  in  nature,  and  which  soften  and  tone 
down  the  harsh  and  saturated  colours.  But  consider- 
ing the  limitations,  the  results  reflect  great  credit  on 
the  inventors. 


Plate  XII, 


The  Kinemacolor  Projector,  showing  Colour  Filter  in  position. 

To  face  p.  210. 


KINEMATOGRAPHY  IN  COLOUR  21 1 


In  preparing  the  rotary  colour-filter  it  is  necessary 
to  bear  in  mind  that  red  is  more  vivid  to  the  eye  than 
green,  so  that  the  balance  of  colour  in  the  two  dyed 
gelatines  must  be  correctly  maintained.  This  is  done 
by  making  the  red  the  standard,  and  adjusting  the 
green  to  equalise  it. 

A single  film  of  red  gelatine  and  one  of  green 
being  fitted  to  the  shutter,  a second  film  of  green 
gelatine  is  added  to  the  first,  trimmed  to  the  requisite 
width,  by  trial,  and  then  affixed  to  it.  As  a rule  it 
is  made  to  occupy  the  middle  third  of  the  gelatine. 
If  it  is  too  narrow  the  green  will  preponderate  and 
yellows  will  have  a greenish  hue;  if  it  is  too  wide 
the  green  will  be  too  dense  and  the  red  will  be  in 
excess,  so  that  yellows  will  have  an  orange  hue.  If 
of  correct  width  the  filter  on  being  rotated  will  show 
a disc  on  the  screen  of  a neutral  white  hue. 

The  perfection  of  the  picture  on  the  screen  is  largely 
due  to  the  correctness  of  the  exposure  and  the  skill  of 
the  photographer.  If  more  than  two  feet  of  film  were 
exposed  per  second  the  objects  would  appear  to  move 
slowly ; if  less,  too  fast.  If  the  exposure  is  too  short 
the  colours  will  be  unnaturally  vivid,  if  too  long  the 
colours  will  be  dull.  On  the  other  hand,  if  the  sub- 
ject is  too  brightly  lighted  the  picture  will  be  white 
or  colourless.  If  too  dark  the  colours  will  be  poor, 
clogged,  and  without  detail. 

It  might  rightly  be  asked,  if  only  red  and  green 
filters  are  used,  how  can  blue  effects  be  produced? 
One  may  either  use  a yellow-green  filter  which  would 
give  the  correct  hue  to  grass  and  foliage,  or  a deep 
blue-green  which  would  give  an  imperfect  colour 


2 12 


PHOTOGRAPHY  IN  COLOURS 


to  grass,  but  which  in  artificial  light  would  give  an 
effect  indistinguishable  from  a pure  blue  itself.  Hence, 
Mr.  Smith  has  made  a compromise  by  sacrificing  a 
little  of  the  purity  of  each.  If  the  green  used  for 
projection  (which,  by  the  way,  is  a different  shade  from 
that  used  for  exposing  the  film)  be  examined  by  the 
spectroscope,  it  will  be  noticed  that  quite  a considerable 
amount  of  blue  passes  through  in  addition  to  the  green. 
In  the  next  place  advantage  has  been  taken  of  the 
effect  of  yellow  light  on  greens.  Now,  if  blue  be 
mixed  with  a very  large  proportion  of  white  light 
such  as  we  find  in  our  Northern  blue  skies,  or  re- 
flected from  large  sheets  of  water,  and  it  be  seen 
through  a green  glass — although  it  appear  greenish 
in  daylight — yet  in  artificial  light  it  will  appear  indis- 
tinguishable from  blue,  and  that  is  what  happens 
when  projected  on  the  screen.  The  blues  of  the  sky 
and  sea  or  rivers  appear  to  the  eye  on  the  screen  as 
greenish-blues,  although  taken  through  a green  filter,  and 
the  effect  is  sometimes  natural  and  pleasing.  Further- 
more the  combination  of  the  red  and  green  lights  gives 
rise  to  the  sensation  of  greenish-yellow,  pure  yellow,  or 
orange,  according  to  the  preponderance  of  green  or  red 
light  in  this  mixture.  In  any  case  there  is  always  a 
trace  of  residual  yellow  and  residual  blue  in  the  high 
lights.  Hence,  the  addition  of  this  residual  blue  to  the 
yellow  will  give  rise  to  white.  This  is  the  explanation 
of  how  the  whites  and  the  blues  are  reproduced  by 
“ Kinemacolor.” 

§ 105a.  Gaumont’s  Method  of  Colour  Cinemato- 
graphy.— Although  this  method  is  the  oldest  of  all,  it 
is  in  the  writer’s  opinion  superior  to  the  Kinemacolor 


*v> 


Plate  XIII. 


To  fact  p.  212. 


Apparatus  arranged  for  Kinemacolor  Projection 


CINEMATOGRAPHY  IN  COLOUR  213 


process,  and  is  briefly  as  follows.  Three  small  cinemato- 
graph pictures  are  taken  with  an  exposure  of  the  1 /30th 
second  through  three  lenses  simultaneously,  each  being 
furnished  with  its  respective  colour  filter,  and  after 
each  exposure  the  film  is  moved  on  over  the  space 
occupied  by  the  three  pictures,  i.e.  about  56  mm.,  or 
roughly  two  and  a quarter  times  as  much  as  for  the 
ordinary  cinematograph  picture.  Projection  is  accom- 
plished by  three  lenses,  each  carrying  its  own  colour 
filter,  which  is  slightly  different  from  the  one  used  in 
taking  the  negative,  and  registration  is  effected  by 
moving  the  top  and  bottom  lenses  in  three  directions 
by  a very  ingenious  mechanism.  This  method  only 
reached  a practical  stage  by  the  production  of  exceed- 
ingly rapid  and  highly  colour-sensitive  emulsions. 


CHAPTER  XIII 


COLOUR  PHOTOMICROGRAPHY 

§ 106.  Permanent  records  of  microscopic  objects  are  so 
essential  that  no  worker  can  afford  to  be  without  a 
photographic  camera,  since  the  old  methods  of  drawing 
objects  through  a camera  lucida  can  never  be  absolutely 
reliable,  and  are  besides  tedious  to  execute.  Objects 
which  require  a high  magnification  to  be  seen,  are 
nearly  always  transparent,  and  can  be  made  out  just 
as  well  in  monochrome,  provided  that  a panchromatic 
(or  Wratten  M plate)  with  a suitable  colour  filter  be 
used.  Still,  since  many  anatomical  and  botanical 
structures  take  on  a selective  action  with  certain  stains, 
it  is  advantageous  to  reproduce  them  in  the  colours 
seen.  Moreover,  when  slides  are  projected  on  to  a 
screen,  the  effect  is  greatly  enhanced  by  colour,  and 
often  details  can  be  shown  which  are  lost  in  mono- 
chrome. 

The  five  methods  at  present  in  use  are  the  Auto- 
chrome, the  Dufay,  the  Paget  screen  plates,  and  the 
three  and  two  separate  plate  methods. 

As  regards  the  choice  of  methods,  the  three  and  two 
colour  separate  printing  methods  are  undoubtedly  the 
most  satisfactory,  but  at  the  same  time  the  most 
tedious,  and  present  far  more  technical  difficulties  to 


COLOUR  PHOTOMICROGRAPHY 


215 


the  amateur  except  in  practised  hands.  They  are 
entirely  free  from  all  traces  of  grain,  and  thus  show  up 
the  finest  detail,  and  by  reason  of  their  great  trans- 
parency form  ideal  slides  for  the  lantern.  Neverthe- 
less, the  single-plate  methods  are  so  simple  that  most 
workers  will  use  nothing  else.  The  Dufay  and  Paget 
plates  are  much  the  best  for  lantern  projection,  but  the 
Autochrome  plate  is  preferred  by  many,  because  the 
colours  are  truer  to  nature,  and  the  losses  through 
failures  are  fewer  than  by  any  other  method.  Since 
the  working  details  of  all  these  processes  are  given 
very  fully  in  other  parts  of  the  book,  they  need  not  be 
referred  to  here. 

It  is  in  opaque  objects  that  colour  is  best  shown,  and 
such  objects  rarely  require  high  magnification,  i.e. 
above  30  or  50  diameters.  Low  power  microphoto- 
graphy presents  far  fewer  difficulties  to  the  tyro  than 
high  power  work,  since  none  of  the  adjustments 
require  anything  like  the  same  precision  in  regulating, 
in  fact,  below  50  diameters  a substage  condenser  will 
rarely  be  needed. 

We  may  divide  the  subject  into  two  heads  : low 
power,  and  high  power  photomicrography. 

§ 107.  Low  power  Photomicrography. — A few 
points  may  be  useful  to  the  reader  regarding  the 
apparatus. 

The  two  most  essential  things  are : first,  to  secure 
rigidity  and  freedom  from  all  vibration  in  all  the  parts ; 
and  secondly,  correct  centering  of  the  axial  rays. 

The  table . — A strong  kitchen  table  answers  every 
purpose.  A couple  of  rails  or  guiding  rods,  made  of 
mahogany,  and  extending  the  whole  length  of  the  table 


2l6 


PHOTOGRAPHY  IN  COLOURS 


on  either  side  of  the  middle  line,  and  about  five  inches 
apart,  should  be  screwed  down. 

The  camera. — Each  end  of  the  camera  should  consist 
of  a square  frame  of  mahogany.  The  front  part  should 
rest  below  on  a broad  wooden  or  metal  support,  which 
fits  exactly  between  the  guides  so  as  to  allow  of  its 
sliding  freely  backwards  and  forwards  without  any  side 
shake.  The  sides  of  the  guides  facing  each  other 
should  have  a rectangular  groove  (or  rabbet)  running 
the  whole  length,  and  each  wooden  support  should  be 
provided  with  a corresponding  tongue  which  should  fit 
the  rabbet.  This  allows  the  supports  to  be  clamped 
in  any  position  along  the  table  by  means  of  a good 
broad  screw,  which  screws  through  the  support  and  can 
be  made  to  press  with  its  flat  end  against  the  table, 
the  counter  pressure  being  made  by  the  tongue  against 
the  rabbet.  Both  the  front  and  back  of  the  camera, 
as  well  as  the  microscope,  water-trough,  screen,  con- 
denser, and  radiant,  should  each  be  provided  with  a 
similar  support,  but  as  the  camera  front  and  back  are 
the  only  parts  liable  to  shift  (owing  to  the  spring  of 
the  bellows),  the  other  accessories  do  not  require  a 
clamp.  The  camera  should  have  at  least  1^  inches  of 
rising  front  which  can  likewise  be  clamped  in  any  posi- 
tion, while  the  back  which  holds  the  focussing  screen 
and  double  slide  should  be  fitted  so  as  to  allow  its  being 
moved  laterally  across  the  support  through  a distance 
of  about  inches  on  either  side  of  the  middle  line, 
and  clamped  in  any  position.  The  use  of  this  will  be 
explained  presently.  The  ocular  end  of  the  microscope 
should  be  connected  with  the  central  aperture  in  the 
front  of  the  camera  (just  inside  the  flange  of  the  lens) 


COLOUR  PHOTOMICROGRAPHY 


217 


by  means  of  a large  metal  collar  which  while  excluding 
all  extraneous  light  allows  of  free  movement  between 
the  microscope  and  camera  without  the  slightest  con- 
tact between  them. 

The  back  of  the  camera  must  be  square  so  as  to 
allow  of  vertical  or  horizontal  pictures  being  taken  at 
pleasure.  My  camera  is  a whole-plate  one,  with 
adapters  for  half-plate,  5x4  and  quarter  plate  sizes. 
This  is  the  apparatus  which  I have  used  continually 
for  many  years  past,  and  I have  never  been  able  to 
improve  upon  it.  It  was  originally  made  from  the 
description  given  in  Dr.  E.  C.  Bousfield’s  book,1  which 
teems  with  original  and  valuable  information. 

The  sliding  support  for  the  microscope  should  have 
a recess  to  hold  each  leg  in  position,  or  if  the  micro- 
scope has  the  German  tuning-fork  shaped  foot,  each 
prong  can  be  clamped  by  a movable  wooden  button. 

My  camera  has  an  extension  of  4 ft.  4 ins.,  which 
just  permits  of  50  magnifications  with  a 25  mm. 
(1  in.)  Protar  Anastigmat  screwed  on  to  the  front. 

Any  ordinary  anastigmat  of  one,  two,  or  three  inches 
focal  length  will  give  just  as  good  an  image  as  the 
best  microscope  objective.  Zeiss  introduced  a series 
of  Planar  objectives  of  25  mm.  and  50  mm.  focal 
lengths,  and  F/4,  5 aperture,  which  are  identical  in 
construction  with  ordinary  photographic  objectives, 
and  can  be  employed  equally  well  for  the  microscope, 
bioscope,  or  snapshot  camera.  They  are  fitted  with 
an  iris  diaphragm,  and  have  the  standard  microscope 
screw.  When  screwed  on  to  the  front  of  the  camera 

1 “ Photo-micrography,”  by  Dr.  E.  C.  Bousfield.  J.  & A. 
Churchill,  London. 


2 I 8 


PHOTOGRAPHY  IN  COLOURS 


enlargements  may  be  photographed  without  a micro- 
scope or  any  other  accessory  apparatus. 

§ 108.  Illumination. — A bright,  even  illumination 
over  the  entire  object  to  be  photographed  is  of  the 
highest  importance.  As  a rule  daylight  can  be  em- 
ployed with  advantage,  but  in  many  cases,  especially 
where  high  relief  or  shadows  are  necessary,  artificial 
light  is  imperative.  The  simplest  method  is  to  place 
an  incandescent  bulb  mounted  on  a stand  on  each 
side  of  the  lens.  It  is  well  to  fix  a mirror  or  tin 
reflector  behind  the  luminant,  so  that  the  light  is  con- 
centrated on  the  object,  and  at  the  same  time  screened 
from  the  lens.  By  using  only  one  light,  or  by  placing 
the  pair  at  different  angles  and  distances  from  the 
object,  the  latter  can  be  thrown  into  any  required 
degree  of  relief  or  shadow.  If  two  60  c.p.  Osmium 
incandescent  bulbs  be  used,  the  exposure  will  be  very 
short  for  unit  magnification.  I find  3 or  4 minutes 
ample  when  using  a Planar  lens,  working  at  F/8.  In 
order  to  ascertain  whether  the  condenser  is  correctly 
centred,  move  the  back  of  the  camera  with  the  screen 
fitted  in  its  place,  until  the  margin  of  the  circle  of 
light  projected  by  the  lens  is  brought  into  view.  Then 
adjust  the  condenser  or  the  radiant,  until  the  margin 
of  the  circle  is  as  sharp  and  free  from  colour  as  possible. 
If  a projecting  eyepiece  is  used,  this  same  method  will 
inform  you  at  once  which  is  the  best  distance  of 
separation  between  the  front  and  back  elements  of 
the  ocular.  Having  made  the  necessary  corrections, 
the  back  of  the  camera  is  swung  round  to  its  original 
axial  position. 

§ 109.  Methods  of  ascertaining  the  Magnifica- 


COLOUR  PHOTOMICROGRAPHY 


219 


tion.— It  is  well  to  remember  that  whatever  the  focal 
length  of  the  lens,  the  camera  must  be  extended  as 
many  focal  lengths  + 1 from  the  lens  as  the  number  of 
magnifications  (m)  required,  while  the  distance  of  the 
object  from  the  lens  will  always  be  the  1/mth  of  that 
distance  + 1.  For  example,  suppose  a magnification 
of  8 times  (diameters)  be  required,  and  the  lens 
chosen  is  of  3-in.  focus.  Then  the  camera  must  be 
extended  (8  + 1)  X 3,  or  27  inches  from  the  optical 
centre  of  the  lens,  while  the  object  must  be  27/8  inches 
from  the  centre  of  the  lens  = 3|  inches.  Again, 
suppose  the  object  is  to  be  taken  natural  size  with 
the  same  lens.  Then  as  the  magnification  = 1,  the 
camera  must  be  extended  1 + 1 times  F,  or  6 inches, 
while  the  object  will  be  1/mth  of  that  distance,  or 
1 + 1/1  times  = 6 in.,  i.e.  the  same  distance  as  before. 
Lastly,  suppose  the  object  is  to  be  reduced  to  J the 
size,  then  the  distances  will  be  the  same  as  in  the 
first  example,  only  image  and  object  will  have  to 
change  places,  so  that  the  object  will  now  have  to  be 
27  inches  away,  and  the  image  3§  inches. 

If  we  require  the  total  magnification  of  the  camera 
extension,  plus  the  magnification  produced  by  the 
microscope,  we  have  to  consider  three  factors. 

First,  we  have  the  initial  magnification  produced  by 
the  objective.  This  is  given  in  all  the  catalogues  issued 
by  the  makers.  It  may  be  easily  reckoned  by  adding 
a nought  to  the  denominator  of  the  focal  length 
expressed  in  inches  or  fractions  of  an  inch,  and  divid- 
ing this  by  the  numerator.  Thus,  in  the  case  of  a 
2-inch  objective,  the  denominator  being  1,  we  have  for 
the  initial  magnification  10  divided  by  2 = 5 times. 


220 


PHOTOGRAPHY  IN  COLOURS 


In  the  same  way  a 1-inch  objective  magnifies  10  times, 
a ^-inch  objective  magnifies  40  times,  a ^-inch 
objective  120  times,  and  so  on.  This  initial  magnifica- 
tion is  again  magnified  by  the  eyepiece,  which  as  we 
have  shown  is  modified  by  the  tube  length  as  measured 
from  the  collar  where  the  objective  screws  in,  to  the 
top  of  the  tube,  or  a little  below  it.  In  most  of  the 
Continental  makes  this  is  160  mm.,  while  most  of  the 
English  models  measure  10  inches,  or  250  mm.  As  a 
rule,  the  magnification  is  marked  on  the  ocular  for  the 
tube  for  which  it  is  to  be  used,  but  if  it  is  to  be  used 
for  any  other  tube  length  the  factor  is 

Actual  tube  length  A 

Standard  tube  length  Focal  length  of  eyepiece 

A being  the  standard  tube-length.  Thus,  if  the  focal 
length  of  the  eyepiece  be  1 inch,  or  25  mm.,  we  shall 
have  a magnification  of  160/25  or  6|  times  for  an 
average  Continental  microscope,  or  250 /25  or  10  times 
when  used  with  an  English  standard  microscope. 
Lastly,  in  order  to  get  the  total  magnification  of  a 
compound  microscope  attached  to  a camera,  it  is 
necessary  to  multiply  all  three  factors  together.  Thus, 
suppose  we  are  using  a ■—  inch  objective  with  an 
inch  ocular  on  a 10-inch  tube,  and  the  camera  is  ex- 
tended 20  inches.  The  factor  for  the  camera  extension 

extension  in  inches  extension  in  mm.  .... 

= n . — r or  — , which  in 

10  inches  250  mm. 

the  above  case  =£  fg  = 2 times.  Hence  the  total  magni- 
fication = 120  X 10  X 2 = 2400  times.  In  order  to 
check  this  it  is  quite  easy  to  measure  the  magnification 


COLOUR  PHOTOMICROGRAPHY 


22  I 


direct.  We  place  a microscope  slide  ruled  in  l/10ths 
and  1 /lOOths  of  a mm.  (which  can  be  obtained  of  any 
dealer)  on  the  microscope  stage,  and  focus  for  one  of 
the  1 /10th  divisions  if  for  a low  power,  or  a 1 /100th  mm. 
if  a high  power.  Suppose  in  the  latter  case  the 
interval  between  two  lines  measures  6-5  mm.  on  the 
focussing  screen  of  the  camera,  then  the  magnification 
is  clearly  100  X 6-5  or  650  times. 

§ 110.  Exposure. — The  correct  exposure  is,  as  we 
have  more  than  once  pointed  out,  of  supreme  import- 
ance. Many  elaborate  calculations  have  been  given  in 
the  text-books  for  arriving  at  the  correct  exposure  by 
multiplying  various  factors  together,  but  the  simplest 
and,  as  a rule,  the  best  and  most  accurate  way,  is  to 
make  a series  of  trial  exposures  on  a portion  of  the 
plates  to  be  used.  It  is  well  worth  while  to  have  a 
special  carrier  made  to  fit  into  the  slide.  A quarter- 
plate  can  readily  be  cut  up  lengthways  in  a subdued 
Yirida  or  Wratten  “ Safe-light  ” into  three  or  four 
equal  strips,  and  one  of  them  placed  in  the  carrier 
after  the  object  has  been  focussed  on  the  ground-glass. 
The  shutter  of  the  slide  should  have  previously  been 
ruled  on  the  inside  with  white  vertical  lines,  say  2 cm. 
apart,  and  numbered  consecutively.  When  the  slide 
is  in  position  draw  out  the  entire  shutter,  and  expose 
for  what  you  would  consider  to  be  about  a quarter  the 
correct  exposure  (say  two  seconds).  At  the  end  of  the 
two  seconds  cap  the  lens,  close  the  slide  2 cm.,  and 
expose  the  rest  of  the  plate  for  another  2 seconds. 
Repeat  the  process  and  expose  for  4 seconds,  again  for 
4 seconds,  and  finally  for  8 seconds.  In  this  way  the 
first  two  centimetres  will  have  had  2 seconds  exposure, 


222 


PHOTOGRAPHY  IN  COLOURS 


the  next  two  centimetres  4 seconds,  the  third  8 seconds, 
the  fourth  12  seconds,  and  the  last  piece  20  seconds 
respectively.  When  the  strip  of  plate  is  developed, 
one  of  the  portions  will  almost  certainly  be  correctly 
exposed.  This  will  be  shown  by  giving  a clear  plucky 
negative,  with  full  details  in  the  half-tones  and  shadows, 
which  latter  should  not  be  too  dense  to  print  easily, 
and  a little  experience  will  at  once  decide  this  when  it 
is  held  up  to  the  light.  Once  the  correct  exposure  is 
known,  any  modification  in  the  factors  which  influence 
it  can  at  once  be  arrived  at  by  calculation. 

§ 111.  Factors  which  Influence  Exposure. — The 
following  are  the  chief  factors  which  influence  exposure  : 

1.  Character  of  the  light. 

2.  Degree  of  magnification. 

3.  Numerical  Aperture  (N.  A.). 

4.  Speed  and  colour  sensitivity  of  plate. 

5.  Colour  and  density  of  screen. 

1.  Character  of  the  light. — The  following  table  gives 
the  approximate  factors  for  the  most  appropriate 
luminants  : — 


Factor  for 

Approximate 

Light  source. 

candle- 

power. 

Panchromatic 
“ M ” plate. 

Ortho- 

chromatic 

plate. 

Ordinary 

rapid 

plate. 

Oil  flat- wick  lamp  . 

15 

1 

4 

8 

Incandescent  gas 

85-60 

1 

3 

3 

4 

4 

3 

Nernst  lamp  (1  amp.) 

200 

1 

12 

1 

5 

1 

3 

Acetylene  .... 

40 

JL 

12 

i 

i 

Direct  arc  (4  amp.)  . 

300 

JL 

50 

35 

53 

Direct  arc  (80  amp.) 

3000 

101)0 

700 

Too 

COLOUR  PHOTOMICROGRAPHY 


223 


Theoretically  the  exposure  is  directly  proportionate 
to  the  candlepower.  As,  however,  different  kinds  of 
luminants  vary  enormously  in  their  richness  in  blue- 
violet  rays,  as  well  as  in  the  amount  of  yellow  light, 
it  is  impossible  to  calculate  off-hand  what  the  exposure 
should  be  with  any  other  kind  of  luminant.  The  above 
table  based  on  practical  experience  will  be  found  very 
useful.  They  are  for  the  most  part  taken  from  the 
“ Kodak  ” pamphlet,  “ Photomicrography,”  issued  by 
the  Kodak  Co.,  Ltd. 

2.  Effect  of  magnification. — The  exposure  varies  as 
the  square  of  the  magnification.  The  following  table 
is  based  on  this  law : — 


Magnification. 

Exposure. 

10 

1 

1 OO 

25 

iV 

50 

1 

4 

100 

1 

250 

6 

500 

25 

1000 

100 

3.  Effect  of  N.  A. — The  exposure  varies  inversely  as 
the  numerical  aperture. 


224 


PHOTOGRAPHY  IN  COLOURS 


Objective. 

Average  N.A. 

Exposure. 

2"  or  50  mm. 

. . 0-15 

. . io 

1"  or  25  mm. 

. . 0-25 

. . 4 

or  16  mm. 

. . 0-85 

. . 2 

or  12  mm. 

. . 0*45 

. . 1} 

i"  or  8 mm. 

. . 0-50 

. . 1 

A77  or  6 mm. 

4 

. . 0-8 

2 

* * 5 

A"  or  4 mm. 

. . 0-85 

1 

• * 3 

A7'  or  8 mm. 

. . 0*9 

• * i 

itoi 

. . J to  A or  A 

apoch.  oil  imm. 

JL77  or  2 mm. 

. . A to  a imm. 

o 4 

. . do.  do. 

4.  Speed  and  colour  sensitivity  of  plate. — These  factors 
are  generally  given  on  the  boxes  and  indicated  by 
H.  and  D.,  Watkins,  or  Wynne.  (See  Appendix.) 

5.  Effect  of  colour  and  density  of  screen. — This  varies 
considerably  with  different  kinds  of  lnminants. 


Exposure  Factor  of  Wratten’s  “ M ” Plates  with 
Various  Screens  and  Luminants. 


Screen. 

Transmission. 

Oil. 

iNernst. 

Arc. 

Incand. 

Gas. 

Acety- 

lene. 

A Scarlet  . . . 

/ red  end  to  line  \ 
\ 590/xfi  j 

3 

6 

6 

6 

5 

B Green  .... 

600-460^ 

12 

12 

12 

12 

12 

0 Blue-violet  . . 

510-400 

25 

16 

12 

12 

12 

E Orange  . . . 

red  end  to  560 

2 

3 

6 

6 

4 

F pure  Red . . . 

„ „ 610 

6 

6 

8 

12 

8 

G strong  Yellow  . 

„ „ 510 

U 

2 

4 

4 

3 

H Blue  . . . . 

K3  for  orthochro. 

540-420 

24 

16 

12 

16 

16 

production  . . 

— 

H 

U 

3 

2 

A and  D deep  Red 
B and  E Yellow- 

red  end  to  640 

60 

90 

240 

240 

120 

green  .... 

560-600 

120 

60 

250 

120 

90 

G and  H pure 
Green  .... 
B and  0 Blue- 

510-540 

1000 

1600 

1600 

1600 

1600 

green  .... 

460-510 

1000 

600 

600 

1000 

800 

D and  H Violet  . 

420-460 

200 

150 

64 

160 

90 

B and  G Green  . 

510-600 

25 

25 

64 

20 

30 

COLOUR  PHOTOMICROGRAPHY  225 


According  to  Messrs.  Hind  and  Randles,  liquid 
screens  of  the  same  light  transmission  have,  as  a rule, 
lower  factors. 

The  way  to  make  use  of  these  tables  is  as  follows : 
If,  after  having  made  a trial  experiment  in  the  way 
previously  indicated  by  giving  a series  of  different 
exposures,  you  merely  require  to  alter  one  of  the  factors, 
say,  the  magnification,  it  resolves  itself  into  a simple 
rule-of-three  sum  to  obtain  the  correct  exposure. 
Thus,  suppose  your  first  magnification  was  twenty-five 
times,  and  you  require  a second  negative  with  a mag- 
nification of  100  diameters,  then,  if  the  correct  expo- 
sure was  3 seconds,  with  an  oil  lamp  having  a factor 
of  1,  with  the  new  magnification  and  employing  incan- 
descent gas  having  a factor  of  the  correct  exposure 


will  be  3 x 


/iooy 

\ 25  / 


X J,  or  16  seconds.  Of  course,  all 


the  other  factors  must  be  dealt  with  in  the  same  way. 

§ 112.  High-power  Photomicrography. — It  is 
assumed  that  the  reader  is  acquainted  with  the  general 
principles  of  high-power  objectives  and  substage  illumi- 
nation, and  they  will,  therefore,  not  be  further  discussed. 
The  microscope,  camera,  and  condenser  system  may  be 
arranged  either  in  the  horizontal  position  or  vertically, 
in  which  case  the  light  is  thrown  vertically  upwards 
along  the  optic  axis  by  the  ordinary  substage  mirror. 

The  advantages  of  the  horizontal  form  are  that  one 
can  sit  comfortably  while  arranging  the  specimen 
under  the  microscope,  and  when  focussing  on  the 
screen.  Further,  that  a very  great  extension  of 
camera  may  be  employed. 

The  objections  are  that  fresh  liquid  preparations 


226 


PHOTOGRAPHY  IN  COLOURS 


are  inadmissible,  and  that  the  viscosity  of  the  cedar 
oil  often  interferes  with  the  final  focussing  when  using 
immersion  lenses,  by  rendering  it  necessary  for  the 
slide  to  be  held  down  by  clips,  which  are  absent  in 
most  mechanical  stages. 

In  most  horizontally  placed  apparatus  there  is  some 
difficulty  in  observing  through  the  microscope.  This 
has  been  eliminated  in  the  large  apparatus  of  Zeiss  by 
the  employment  of  two  separate  tables,  one  for  the 
microscope  and  illuminating  systems,  and  one  for  the 
camera. 

The  vertical  form  of  apparatus  has  the  advantage 
when  photographing  liquid  preparations,  and  it  obvi- 
ates the  necessity  of  swinging  the  instrument  into  the 
horizontal  position  every  time  it  is  used  for  photo- 
graphy. Also  the  slide  will  remain  in  focus  by  its  own 
weight  and  the  ordinary  clamps  of  the  stage.  I find 
that  most  of  the  fine  adjustments  retain  their  foci 
better  when  the  instrument  is  vertical.  Since  the 
vertical  camera  is  usually  short,  the  fine  and  coarse 
adjustments  can  be  used  without  any  extending  rods 
or  gearing  for  the  purpose  of  focussing.  The  inevit- 
able drawbacks  to  the  vertical  apparatus  are  first  that 
a camera  of  limited  length  is  almost  compulsory.  If 
one  wishes  to  obtain  a magnification  of,  say,  one  thou- 
sand diameters,  using  a 2-mm.  objective  and  a No.  4 
eyepiece,  the  camera  screen  must  be  about  50  cms. 
from  the  plane  of  the  projection  ocular,  a distance 
which  is  only  just  within  reach  of  the  fine  adjustment 
screw  when  the  operator  is  observing  the  image  on  the 
screen.  Again,  it  is  much  less  fatiguing  to  focus  when 
the  image  is  vertical  than  when  it  is  in  a horizontal 


COL  0 UR  PHO TOMICROGRA PHY  227 

position,  especially  when  the  head  is  enveloped  in  a 
focussing  cloth. 

Bearing  these  things  in  mind,  I felt  convinced  that 
the  ideal  apparatus  for  high-power  work  lay  in  the 
combination  of  a vertically  placed  microscope  with  a 
horizontal  camera.  It  is  true  that  such  an  arrangement 
can  be  contrived  with  the  most  recent  form  of  the 
large  Zeiss  apparatus,  in  which  the  camera  can  be 
racked  up  to  a sufficient  height  to  meet  the  eyepiece  of 
the  vertically  placed  microscope  ; but  the  whole  appa- 
ratus is  not  only  very  expensive  and  cumbersome,  but 
is  of  such  a length  that  a fairly  large  room  is  required 
to  use  it  with  comfort.  This  apparatus  also  suffers  in 
common  with  all  other  horizontal  arrangements  by  the 
necessity  of  a gearing  for  the  transmission  of  the  fine 
adjustment  motion  to  the  operator’s  hand  when  focus- 
sing the  image.  However  perfect  the  gearing  may  be, 
it  cannot  be  trusted  to  keep  in  focus  while  the  dark 
slide  is  being  introduced,  or  until  the  exposure  is  com- 
plete. After  having  constructed  and  experimented 
with  the  two  forms  of  reflex  apparatus,  I have  adopted 
the  following  arrangement,  which  I have  found  most 
satisfactory.  Fig.  27  shows  the  apparatus  diagram- 
matically.  A A'  is  a strong  table  25  mm.  (10  inches) 
wide  (shaded  in  the  diagram).  C is  an  asbestos-lined 
cover  with  changeable  circular  diaphragms  in  front. 
D is  the  usual  condenser  which  collects  the  light  to  a 
point  close  to  the  mirror  of  the  microscope.  F is 
another  stand  to  carry  any  further  fittings,  such  as  an 
auxiliary  lens,  or  a trough  with  colour  filter  solution, 
etc.  BB'  is  a strong,  rigid  table,  carrying  G the 
microscope  in  a vertical  position,  upon  a strong  stool, 


228 


PHOTOGRAPHY  IN  COLOURS 


haying  at  least  3 
clearance,  together 

B 


inches  clear  space  below  it.  This 
with  the  fact  that  the  stool  top  is 

B' 


*!  f 


JD 

jhii: 


* 25cm~  " — > 


no  Milled  head  for  raising  or  lowering  stage. 

Alu-mini-uin.  rod  uilth  delicate,  chetch  to 
grife  -milled  not-  of  Berger  f roe,  Toollon.. 
(Shaded  Irlceck.^ 

S,s  Sc  Sis.  Reds  Svcjiporting  Carreer  ce 
Jv.Je  Path  of  rags  lo  TMcroscofoe.  mirror. 
A . Radiard  ere  hood 

1 Blackened  tide  making  light- btght 
Joint 7 with  hood  of  retmrseng  prism. 

The.  dotted,  circle  shews  area  covered 
Ing  Veil  Jar  -xesed  to  protect  microscopic. . 

TtCJ}.  Severn.  cUt> 


Fig.  27. 


quite  open  between  the  feet  of  the  microscope,  will  be 
explained  later.  H is  a mount  holding  a small  revers- 
ing prism  with  silvered  hypotenuse  at  45  degrees  to 
the  two  optic  axes,  and  provided  with  a blackened 


Plate  XIY. 


[7'o  face  p.  '228, 


Another  view  of  the  central  part  of  the  apparatus  for  photomicrography. 


COLOUR  PHOTOMICROGRAPHY 


229 


hood  which  makes  a non-contact,  light-tight  junction 
with  the  camera  front.  J J'  is  the  camera  made  to 
slide  on  two  round  bars,  so  that  it  may  be  bodily  with- 
drawn to  a short  distance  from  the  microscope. 

Another  view  of  the  central  part  of  the  apparatus  is 
shown  in  Plate  XIY.  The  camera  is  drawn  back  from 
the  prism  hood  so  as  to  show  the  rod  leading  to  the 
slow  motion  behind  the  camera.  In  the  foreground, 
on  the  optical  bench,  is  an  extra  iris  diaphragm  as 
well  as  a tank  for  light-filtering  solutions.  The  ordi- 
nary mirror  of  the  microscope  is  in  position. 

This  arrangement  presents  at  least  two  great  advan- 
tages. Firstly,  the  operator  is  enabled  to  sit  com- 
fortably at  the  point  B,  and  perform  any  focussing  or 
other  adjustments  with  the  microscope,  thereby  dis- 
pensing with  a complicated  revolving  table,  etc. 

Secondly,  a straight,  light  rod  of  aluminium  or  other 
metal  can  be  instantly  set  in  action  with  the  small 
milled  head  of  the  Berger  or  other  fine  adjustment. 
It  requires  for  this  purpose  only  a very  simple  fitting. 
All  gearing  for  transmitting  the  fine  motion  is  there- 
fore done  away  with,  and,  if  only  a small,  cloth-lined 
Y support  be  fitted  near  the  observer’s  hand,  the  rod 
need  not  be  lifted  or  lowered  at  all.  Owing  to  the 
vertical  position  of  the  microscope,  and  the  consequent 
elevated  position  of  the  camera,  the  aluminium  rod 
passes  beneath  the  camera  to  the  right  of  its  supports. 

It  will  be  found  advantageous  to  have  a cloth-lined 
gutter  arranged  alongside  the  apparatus  to  hold  the 
rod  when  not  in  use,  so  that  it  may  not  sag  or  bend 
when  at  rest.  A further  advantage  of  this  contrivance 
is  that  the  microscope  can  be  protected  in  a moment 


230  PHOTOGRAPHY  IN  COLOURS 


when  work  is  finished  by  simply  covering  it  up  with  a 
bell-jar  as  it  rests  on  the  stool.  The  3-inch  clearance 
below  the  latter  is  to  enable  the  first-surface  mirror  to 
reflect  the  light  upwards.  This  enables  a more  perfect 
reproduction  of  the  original  beam  of  light  to  be  formed, 
and  also  permits  of  longer  patterns  of  aplanatic  con- 
densers to  be  inserted  below  the  stage.  Further,  it 
allows  of  room  for  the  insertion  of  an  arrangement  for 
the  quick  changing  of  colour  filters.  It  is  sometimes 
useful  to  arrange  the  three-colour  screen  of  a Sanger- 
Shepherd  repeating-back  over  the  foot  of  the  micro- 
scope, especially  if  it  has  a horseshoe  stand.  If  a 
mirror  be  adapted  below  the  stool  it  may  be  fixed 
exactly  at  45,  so  that  the  light  will  remain  per- 
manently centred.  Of  course,  in  this  case  the  optical 
bench  must  be  correspondingly  lowered.  The  whole 
arrangement  can  easily  be  constructed  by  any  one 
with  a little  mechanical  skill,  the  only  precaution  to 
be  observed  being  that  the  camera  supports  at  either 
end  must  be  narrow  enough  to  enable  the  slow-motion 
rod  to  pass  straight  on  to  the  milled  head.  The  rods 
which  slide  in  supports  with  the  camera  are  easily 
made  from  brass-plated  curtain  rods,  but  a solid  bar 
of  square  or  prism-shaped  section  would  be  preferable 
to  the  two  round  rods.  The  only  objection  to  the  form 
of  apparatus  above  described  is  that  there  is  no  way 
of  varying  the  height  of  either  the  microscope  as  a 
whole,  or  of  the  camera,  to  allow  for  the  difference  of 
focus  of  different  lenses.  This  is  not  a serious  objection 
at  all,  and  can  be  easily  surmounted  by  having  a front 
to  the  camera  which  can  be  raised  or  lowered  within 
a small  distance,  or  else  by  slightly  withdrawing  or 


COLOUR  PHOTOMICROGRAPHY  23  I 


closing  the  drawtube,  and  at  the  same  time  correcting 
the  magnification  by  lengthening  or  shortening  the 
camera.  A still  better  way  is  to  employ  a microscope 
like  the  Zeiss  pattern  I S,  in  which  the  stage  itself  can 
be  racked  up  and  down  independently  of  the  rest  of 
the  microscope.  This  instrument  is  very  solidly  built, 
and  for  this  reason  is  eminently  suited  for  photo- 
micrography, and  it  is  to  be  hoped  that  British 
manufacturers  will  furnish  some  analogous  form  of 
stand. 

Details  relating  to  high-power  colour  photography. — 
The  majority  of  high-power  colour  work  consists  of 
pathological  and  histological,  and  especially  bacterio- 
logical preparations.  Passing  on  to  the  actual  subject 
of  making  photomicrographs  in  colour,  I take  it  that 
the  object  of  the  great  majority  of  operators  in  this 
field  is  not  to  produce  transcendental  ingenuities  in 
colour  for  their  own  sake,  but  to  represent  as  accurately 
as  possible  any  microscopical  preparations  which  they 
wish  to  place  on  record,  or  to  demonstrate  with  the 
lantern.  I shall  write,  therefore,  from  the  point  of 
view  of  the  bacteriologist  and  pathologist.  The 
majority  of  the  preparations  which  a bacteriologist 
will  wish  to  accurately  represent  are  slide  or  cover- 
glass  preparations  of  cultures  or  secretions  stained 
in  one  colour.  The  colours  will  be  either  blue 
(methylene  blue),  violet  (gentian  or  methyl  violet), 
or  red  (fuchsine). 

(1)  The  preparation  is  stained  with  methylene  blue. 
Let  us  suppose  a magnification  of  1000  diameters  is 
required.  We  take  a rapid  isochromatic  plate  (Wrat- 
ten’s  isochromatic  is  strongly  recommended  for  this 


232 


PHOTOGRAPHY  IN  COLOURS 


kind  of  work).  Use  a deep  orange  screen  or  a scarlet-red 
screen  (Wratten’s  “ G ” screens  superimposed  answer 
well).  Select  a 2-mm.  oil  immersion  apochromat  or 
semi-apochromat  of  1,4  or  1,3  N.  A.,  and  a No.  2 
projection  ocular.  (By  No.  2 we  mean  an  ocular 
giving  two  magnifications.)  The  camera  must  be 
racked  out  to  a distance  of  100  cm.  from  the  hypo- 
teneuse  of  the  inverting  prism  to  the  focussing  screen. 
Of  course,  if  a No.  4 ocular  be  used,  the  camera  must 
be  racked  out  50  cm.  It  is  a good  plan  to  have  a 
scale  fixed  along  the  whole  length  of  the  table  marked 
out  in  inches  and  centimetres. 

Exposure . — The  tables  for  finding  the  correct  ex- 
posure have  already  been  given  under  the  heading 
of  low-power  photography,  and  they  apply  equally 
well  in  this  case.  I may  add,  however,  that  with  a 
radiant  of  about  750  cp.,  which  I obtain  with  a star 
pattern  triple  filament  (thick  type)  Nernst  lamp,  the 
exposure  would  be  about  90  seconds  for  a bacterial 
culture.  In  all  photographs  of  this  nature  the  object 
is  to  get  as  great  a contrast  as  possible,  and  in  the 
negative  the  bacteria  should  appear  as  nearly  clear 
glass,  and  the  background  as  black  as  possible.  We 
must  therefore  employ  a contrast  or  hard  developer. 
For  this  purpose  I have  found  the  following  developer 
very  efficient : — 


Solution  A. 

Hydroquinone  . . 9 5 grms. 

Sod.  sulphite  . . 50  grms. 

Citric  acid  . . . 3-5  grms. 

Pot.  bromide  . . 3 grms. 

Water 500  c.c. 


Solution  B. 

Sod.  hydrate  (pure)  90  grms. 
Water 500  c.c. 


COLOUR  PHOTOMICROGRAPHY  233 

To  develop  take  equal  parts  of  A and  B and  add  from 
half  to  an  equal  part  of  water. 

Push  development  to  the  full,  and  fix  thoroughly 
with  acid  hypo.  It  is  always  easy  to  clear  up  with 
hypo  and  ferricyanide  of  potash,  by  which  the  contrast 
will  at  the  same  time  be  increased.  The  negative 
having  been  obtained,  make  a clean  positive  on  a 
lantern  plate,  clear  if  necessary,  so  that  the  background 
is  perfectly  clear  glass.  Wash  thoroughly,  and  tone 
with  the  following  toning  solution  : — 

Potassium  ferrocyanide  . . 28  grams 

Water 280  c.c. 

Bleach  the  plate  in  this,  and  wash  for  ten  minutes. 
Then  place  in  Sanger-Shepherd’s  “ Minus  Red  stain- 
ing solution,”  1 part  to  2 parts  water  for  1J  minutes. 
Transfer  to  hypo  (1  to  5).  Keep  on  applying  fresh 
hypo  until  a clear  blue  image  is  obtained.  Clear  if 
necessary  in  sulphuric  acid  (1  to  300  water).  Any 
other  good  blue  formula  may  be  used.  Then  wash 
well. 

Now,  if  the  original  preparation  had  been  stained 
with  carbol-methylene  blue,  it  will  tend  to  be  of  a 
greenish-blue  colour,  and  the  lantern  plate  in  its 
present  stage  will  probably  be  a very  accurate  re- 
production of  it.  If,  however,  the  stain  was  an 
alkaline  methylene  blue  (Loeffler),  it  will  be  more  of 
a true  spectrum  blue.  To  obtain  this  the  lantern 
plate  should  be  dried,  then  soaked  in  distilled  water, 
and  flooded  with  very  dilute  nitric  acid,  and  then 
again  thoroughly  washed.  If  there  is  any  blue  in 
the  background,  a very  rapid  treatment  with  dilute 


234 


PHOTOGRAPHY  IN  COLOURS 


potassium  oxalate  solution,  followed  by  thorough 
washing,  will  remove  it. 

(2)  In  the  case  of  a violet-stained  film,  proceed 
exactly  as  before,  excepting  that  the  light-filter  may 
be  of  a lighter  orange  colour,  without  any  green. 
One  may  use  either  two  or  three  superimposed 
Wratten  “ G ” filters,  or  a trough  containing  bi- 
chromate of  potash  solution  of  corresponding  depth 
of  colour,  according  to  the  intensity  of  the  violet 
stain.  Take  a flat  celluloid  gelatine-coated  film, 
sensitise  in  ammonium  bichromate,  or  in  the  Sanger- 
Shepherd  film-sensitising  salt,  and  dry  in  the  usual 
way,  out  of  reach  of  dust  and  light.  Then  print  this 
out  with  the  uncoated  slide  against  the  film  of  the 
negative.  Expose  until  details  are  visible  as  a pale 
silver  image,  or,  better  still,  against  a Chapman- Jones 
“ Fraction  tint  actinometer.”  Expose  simultaneously 
to  the  extent  shown  to  be  correct  by  previous  trials. 
Then  develop  the  film  with  warm  water  and  dry. 
Make  a solution  of  methyl  or  gentian  violet  (methyl 
violet  6B),  called  crystal  violet,  answers  well.  A 0-5 
per  cent,  solution  of  crystal  violet  added  to  100  c.c.  of 
distilled  water.  Stain  the  film  in  this,  and  then  wash 
until  the  bacteria  appear  deeply  stained  on  a colourless 
ground.  When  dry,  varnish  with  Sanger- Shepherd 
film  varnish.  The  film  will  look  better  and  clearer  if 
mounted  quite  dry  and  warm  between  lantern  cover- 
glasses  in  melted  hard  neutral  Canada  balsam.  Wait 
until  the  balsam  is  quite  dry,  and  then  paint  round 
the  edge  with  hard  asphaltum  varnish.  Bind  up  as 
usual.  If  it  is  desirable  to  add  a circular  mask  in 
imitation  of  the  microscopic  field,  which  certainly  adds 


COLOUR  PHOTOMICROGRAPHY  235 


to  the  reality  of  the  image,  then  omit  the  balsam  on 
the  celluloid  side  of  the  print,  and  insert  the  mask 
between  the  cover-glass  and  this  side,  after  the  balsam 
of  the  film-side  is  dry. 

(3)  In  the  case  of  a red-stained  preparation  such  as 
a film  of  B.  Typhi , or  Vibrio  Cliolerse  stained  with 
fuchsine,  a green  screen  must  be  used  for  making  the 
negative,  and  the  positive  film  must  be  stained  in  a 
solution  of  fuchsine.  A suitable  solution  is  made  by 
adding  4 c.c.  of  a 05  per  cent,  solution  to  100  c.c.  of 
distilled  water.  Sometimes  in  examining  a fuchsine- 
stained  slide  by  daylight,  the  red  colour  may  not 
appear  so  brilliant  as  that  of  the  original  preparation. 
This  is  because  the  dye  fuchsine,  or  magenta,  transmits 
a varying  amount  of  blue  in  addition  to  the  red  rays, 
according  to  the  makers’  formulae,  or  the  manner  of 
making  the  solutions.  The  yellow  rays  of  the  lantern 
will  largely  correct  the  slight  proportion  of  violet  in 
the  red  image.  If  further  correction  be  desired,  a 
little  eosine  (yellow  shade)  may  be  added  to  the 
fuchsine  solution  used  for  staining  up.  Lantern  slides 
made  as  described  above,  when  looked  at  through  a 
blackened  tube  by  transmitted  light,  give  a remarkably 
exact  reproduction  of  the  picture  seen,  when  the 
original  preparation  is  examined  under  the  micro- 
scope. Hitherto,  only  single-stained  preparations 
have  been  considered,  and  it  is  very  desirable  to 
master  the  making  of  these  before  passing  on  to 
double  and  triple-stained  objects. 

A good  simple  example  of  double-stained  high-power 
work  is  a film  of  tubercular  sputum  stained  by  the 
Ziehl-Neelsen  method.  In  this  process  the  tubercle 


236  PHOTOGRAPHY  IN  COLOURS 

bacilli  appear  as  brilliantly  red  minute  rod-shaped 
objects,  while  the  pus  cells,  debris,  and  other  bacteria 
are  stained  blue.  Put  two  Wratten  “ M ” plates  into 
a dark  slide,  and  expose  one  of  them  under  a green 
filter,  and  the  other  using  a red  filter.  The  first  nega- 
tive will  show  the  tubercle  bacilli  very  clearly  marked, 
and  must  be  printed  on  a gelatine  film  and  stained  up 
with  fuchsine  as  directed  for  single-stained  red  pre- 
parations. The  other  plate  will  show  only  a faint 
image  of  the  bacilli,  but  very  distinct  pus  cells.  From 
this  negative  a black  lantern  plate  should  be  made, 
and  toned  as  described  for  blue-stained  preparations. 
As  the  gelatine  film  was  printed  through  the  back,  if 
the  two  are  placed  face  to  face  in  register,  a very  good 
representation  will  be  the  result. 

The  following  are  the  details  of  a successful  two- 
colour  slide  of  tubercular  sputum  stained  by  the 
Ziehl-Neelsen  method  on  a slide  1 mm.  thick. 

Magnification. — 1000-2  mm.  Apochromatic  oil  im- 
mersion. 

Condenser. — Centering  achromatic  N.  A.  1*0  when 
used  dry — used  here  oiled  to  slide. 

Radiant. — Nernst  lamp.  Large  projector  3 filament, 
star  pattern,  220  volt — continuous. 

1st  “ M ” plate,  Wratten  B screen,  90  seconds  ex- 
posure. 

2nd  “ M ” plate,  Wratten  A screen,  80  seconds  ex- 
posure. 

Developed  with  hydroquinone  in  total  darkness  for 
4 minutes.  There  was  a good  deal  of  development  fog 
which  was  cleared  up  by  Farmer’s  reducer,  resulting 
in  very  good  negatives,  except  that  there  was  a faint 


COLOUR  PHOTOMICROGRAPHY  237 


image  of  the  tubercle  bacilli  on  the  second  plate.  This, 
it  was  found,  could  be  eliminated  by  using  a liquid 
screen,  in  addition  to  the  Wratten  filter. 

This  liquid  filter  was  made  by  adding  07  c.c.  of  a 
0-5  per  cent,  solution  of  diamant  fuchsine  to  230  c.c.  of 
water,  and  the  thickness  of  the  layer  of  fluid  was  3 cm. 
The  theoretical  photographer  may  criticise  the  above 
results,  in  that  a perfect  negative  was  not  at  once 
obtained,  and  that  accurate  spectroscopic  observations 
would  have  eliminated  the  necessity  for  a repetition  of 
the  second  plate.  Actual  work  with  high  powers  will 
soon  convince  any  one  that  practice  often  renders 
theory  nugatory  in  colour  microphotography,  at  any 
rate  in  its  present  stage  of  development.  This  is 
especially  true  in  the  matter  of  exposure  under  very 
high  magnifications.  An  exposure  of,  say,  a simple 
violet  preparation  under  such  conditions  as  I have 
indicated  above,  would  work  out  under  the  theoretical 
formulae  at  something  like  600  seconds,  whereas  in 
practice  an  exposure  of  80  or  90  seconds  will  generally 
yield  a good  negative,  providing  development  is  pro- 
perly conducted.  The  study  of  absorption  spectra  is 
not  only  desirable  for  any  one  wishing  to  excel  in 
colour  photography,  but,  after  working  out  all  the  con- 
ditions for  any  given  preparation,  the  practical  worker 
will  often  retreat  to  the  primitive  refuge  of  placing 
certain  colour  filters  before  his  microscope  condenser 
in  succession,  being  guided  by  the  visual  results  thus 
obtained.  One  must,  of  course,  do  this  under  the 
same  light  to  be  employed  in  taking  the  photograph. 

In  photographing  sections  stained  by  the  old- 
fashioned  methods  of  hsematoxylin  and  eosin,  great 


238  PHOTOGRAPHY  IN  COLOURS 

difficulties  will  arise  when  one  wishes  to  reproduce  the 
colours  with  any  exactitude.  There  is  a table  given  in 
Messrs.  Wratten  and  Wainwright’s  pamphlet  on 
“Photomicrography,”  which  gives  the  absorption 
spectra  for  three  different  hematoxylin  formulae ; but 
all  hematoxylin  and  hematein  solutions  change  as 
they  are  kept  in  vitro,  and  sections  stain  all  kinds  of 
different  shades,  according  to  the  alkali  used  to  blue 
the  preparation,  and  vary  from  other  causes.  It  is  not 
of  vast  importance  that  a lantern  slide  of  a hema- 
toxylin-eosine  preparation  should  show  pure  blue  and 
pink,  and  one  may  therefore  adopt  an  artifice  as  fol- 
lows : take  the  negative  of  a blue  and  pink  hasma- 
toxylin  section  through  a red  or  red  plus  orange  screen, 
then  develop,  and  print  a black  lantern  plate.  Tone 
the  lantern  plate  as  pure  blue  as  possible,  and  stain 
up  afterwards  in  weak  eosin  or  erythrosin. 

The  tout  ensemble  is  generally  quite  sufficiently 
realistic  and  educative,  eosin  being  a diffuse  counter- 
stain at  the  best,  and  only  fitted  for  blood  studies.  It 
is  far  otherwise  when  a section  is  stained  by  some 
method  which  entails  delicate  selective  qualities. 
Suppose  a section  be  stained  first  with  Unna’s 
polychromic  methylene  blue,  then  treated  with  differ- 
entiating agents,  fixed  and  counterstained  with  orange- 
tannin  mixture,  and  finally  counterstained  with  acid 
fuchsin  (Rubin  S.).  In  the  epithelial  cells  one  may 
have  a very  red-violet  tint,  which  becomes  a peacock 
blue  in  certain  secreting  cells,  while  red  blood-cor- 
puscles become  a bright  orange,  and  the  connective 
tissues  a whole  gamut  of  brilliant  tints,  to  say  nothing 
of  accidental  greens  caused  by  the  yellow  elements  in 


COLOUR  PHOTOMICROGRAPHY  239 


the  tissue  combining  with  the  blue  of  the  stain.  In 
such  cases  it  is  obviously  necessary  to  resort  to  one  of 
the  three-colour  processes;  but,  as  has  been  pointed 
out,  high-power  work  is  seldom  required  for  anatomical 
or  pathological  sections — at  least,  not  over  five  hundred 
magnifications ; but  even  with  these  magnifications,  no 
one  plate  method  will  do.  The  grain  of  the  plate  is 
too  coarse.  Hence,  for  all  such  work,  one  of  the  three 
separate-plate  methods  is  far  superior.  For  the 
beginner  it  is  much  simpler  to  adopt  the  Sanger- 
Shepherd  process  as  detailed  in  the  booklet  issued  by 
the  firm.1  All  the  necessary  apparatus  can  also  be 
obtained  from  them  for  their  processes,  as  well  as  for 
most  of  the  other  three-colour  processes.  It  will  be 
found  that  a plate  9 inches  X 3 inches  is  quite  large 
enough  to  make  lantern  slides  from,  and  very  little 
carpentering  ability  is  required  to  adapt  the  whole 
arrangement  to  the  back  of  the  micrographic  camera. 
Instead  of  finding  the  exposure  ratios  through  the 
three  screens  by  means  of  crumpled  wool  or  a plaster 
cast,  one  must  employ  a black  and  white  microscopic 
preparation,  such  as  one  of  the  old-fashioned  micro- 
photographs of  engravings,  or  part  of  a lantern  slide 
diagram,  or  (for  high  powers)  a stage  micrometer. 
When  the  ratios  have  been  found  for  any  particular 
light  and  plate,  it  is  only  necessary  to  carefully  follow 
the  directions  given  in  the  booklet  to  obtain  the  finest 
slides  possible.  But  it  cannot  be  too  much  insisted  on 
that  the  instructions  issued  by  the  company  should  be 

1 “ Working  Instructions  for  the  Sanger-Shepherd  Process  of 
Natural  Colour  Photography.”  Sanger,  Shepherd  and  Co., 
5 and  6,  Gray’s  Inn  Passage,  Red  Lion  Street,  London,  W.C. 


240 


PHOTOGRAPHY  IN  COLOURS 


rigidly  adhered  to,  for,  although  many  of  the  instruc- 
tions may  appear  frivolous  to  the  ordinary  worker,  the 
infringement  of  any  one  of  them  is  apt  to  entail  much 
disappointment  and  loss  of  time.  In  my  opinion,  the 
final  cementing  of  the  glass  positive,  the  two  celluloid 
positives,  and  the  cover-glass  is  the  most  troublesome 
and  tedious  of  all  the  stages,  and  the  amateur  must 
not  be  discouraged  if  he  should  fail  at  first  in  doing  it 
to  his  satisfaction. 

In  conclusion,  we  would  strongly  recommend  the 
following  works  on  this  subject,  which  we  constantly 
refer  to  ourselves  : — 

(1)  Spitta’s  “ Photomicrography.” 

(2)  Article  by  Dr.  Duncan  Eeid,  British  Jour.  Phot. 
Almanac  for  1915. 

(3)  “ Photomicrography,”  issued  in  pamphlet  form 
by  Messrs.  Wratten  and  Wainwright,  Ltd.,  Croydon, 
England. 

(4)  “ Photomicrography  ” (Pamphlet).  Kodak,  Ltd., 
Kingsway,  London,  W.C. 

(5)  “ Photomicrography,”  by  Dr.  E.  C.  Bousfield. 
J.  & A.  Churchill,  London. 

(6)  “ The  Photography  of  Coloured  Objects,”  by 
C.  E.  Kenneth  Mees,  D.Sc.  Lond.,  published  by  The 
Eastman  Kodak  Co.,  Eochester,  New  York. 

(7)  “ Handbook  of  Photomicrography,”  by  Messrs. 
Hind  and  Eandles.  Eoutledge  and  Co.,  London,  1915. 


CHAPTER  XIV 


ART  IN  COLOUR  PHOTOGRAPHY 

§ 113.  What  constitutes  Art  ? — Unlike  science,  art 
is  not  governed  by  well-ascertained  laws,  but  is  largely 
a matter  of  education  and  individual  taste.  Of  course 
it  has  its  rules,  and  is  governed  by  well-established 
principles ; but  when  we  come  to  special  cases  there 
is  room  for  much  argument  and  difference  of  opinion. 
In  fact,  the  only  branch  of  pictorial  art  which  is  based 
on  rigid  and  undisputed  rules  is  that  of  perspective. 
The  more,  therefore,  we  go  into  detail  the  more  we 
must  expect  hostile  criticism.  Before  dealing  with  art 
itself,  it  is  necessary  to  have  a clear  understanding  as 
to  what  we  understand  by  colour,  with  its  various 
hues  and  shades  as  applied  to  art. 

§ 114.  Primary,  Secondary,  and  Tertiary 
Colours. — A long  time  ago  Sir  David  Brewster  pointed 
out  that  instead  of  Newton’s  seven  colours,  white 
light  could  be  resolved  into  three,  viz.  red,  blue,  and 
yellow,  and  for  this  reason  they  were  termed  the  three 
primary  colours.  Each  was  said  to  be  the  complement 
of  the  other  two.  Thus,  red  was  said  to  be  the  com- 
plement of  blue  and  yellow,  yellow  the  complement  of 
red  and  blue,  and  blue  the  complement  of  red  and 
yellow.  A mixture  of  any  two  was  called  a secondary 


242 


PHOTOGRAPHY  IN  COLOURS 


colour.  Thus,  green  was  a secondary  of  yellow  and 
blue,  and  violet  a mixture  of  red  and  blue.  Conse- 
quently a mixture  of  all  three  in  various  proportions 
formed  a tertiary  colour.  Scientifically  this  theory  is 
entirely  wrong,  and  this  becomes  very  obvious  when 
dealing  with  spectral  coloured  light,  although  true  in 
the  case  of  pigments.  Hence,  it  is  very  convenient, 
and  even  necessary,  when  dealing  with  pigments. 
For  instance,  pigments  mixed  in  certain  proportions 
will  form  various  shades  of  grey,  which  coloured  lights 
never  do.  This  fact  is  a most  important  point  to 
remember  in  water-colour  painting,  and  in  retouching 
prints  made  from  photographs  by  the  three-plate 
method,  or  process  blocks,  and  it  will  be  found 
extremely  useful  in  forming  shadows  over  coloured 
parts.  It  was  formerly  believed  that  if  pigments 
could  only  be  obtained  absolutely  pure,  that  a pure 
white  could  be  formed  by  mixing  the  three  primaries 
in  certain  proportions.  As  a matter  of  fact,  this  can 
never  be  done,  since  in  the  sense  formerly  attributed 
to  the  word,  every  colour  is  a primary,  i.e.  every 
spectral  coloured  light  can  be  mixed  with  some  other 
coloured  light  which  will  produce  white  light.  The 
following  are  the  fundamental  complementary  pairs  of 
coloured  lights : — 


Red 

and 

Bine-green  (sea-green) 

Yellow-orange 

and 

Blue  (cyan  blue) 

Yellow 

and 

Violet-blue  (indigo-blue) 

Greenish-yellow 

and 

Violet 

Green 

and 

Purple  (red  and  violet). 

Besides  these  fundamental  complements,  there  are 
subsidiary  complements,  which  are  formed  by  pairing 


ART  IN  COLOUR  PHOTOGRAPHY  243 


intermediate  shades  of  colour.  We  can  illustrate  this 
in  a very  convenient  way  by  means  of  a chromatic 
circle.  In  this  the  colours  of  the  spectrum  are  sub- 
divided, so  that  twenty-four  hues  are  shown  in  all 


Fig.  28. — Chromatic  circle  showing  the  complementary  pairs  of 
colours,  which  are  indicated  by  the  same  numbers. 


which  may  be  arranged  in  a circle  corresponding  to 
their  actual  position  in  the  spectrum.  In  this  circle  it 
will  be  found  that  each  colour  is  exactly  opposite  to  its 
complementary,  i.e.  180°  away  from  it. 

Although  any  two  complementaries  will  produce  the 


244 


PHOTOGRAPHY  IN  COLOURS 


sensation  of  white,  it  is  not  a true  white,  for  Helmholtz 
pointed  out  long  ago  that  the  only  two  colours  which 
would  produce  an  absolutely  pure  white  were  yellow 
(or  yellow-orange)  and  blue  in  certain  proportions.  If 
you  examine  the  colour  diagram  you  will  observe  that 


Violet. 

Indigo- 

blue. 

Cyan- 

blue. 

Blue- 

green. 

Green. 

Greenish- 

yellow. 

Yellow. 

Red 

Purple 

Dark 

Rose 

Light 

Rose 

White 

Whitish- 

yellow 

Gold- 

yellow 

Orange 

Orange 

Deep 
. Rose 

Light 

Rose 

White 

Light 

Yellow 

Yellow 

Yellow 

Yellow 

Light 

Rose 

White 

Light 

Green 

Light 

Green 

Greenish- 

yellow 

Greenish- 

yellow 

White 

Light 

Green 

Light 

Green 

Green 

Green 

Light 

Blue 

Sea- 

blue 

Blue- 

Green 

Blue- 

green 

Sea- 

Blue 

Sea- 

blue 

Cyan-blue 

Indigo 

Blue 

Fig.  29. — Diagram  showing  the  effect  of  Spectral  Colour  Fusion 
after  v.  Helmholtz. 


green  is  the  only  colour  which  cannot  find  a partner  to 
form  white,  or  even  an  imperfect  white,  since  the  com- 
plementary colour  to  green  is  purple,  which  is  a mix- 
ture of  red  and  blue,  and  not  a primary  at  all. 

§ 115.  As  regards  colour  in  art,  we  have  to  dis- 
tinguish between  hue,  tint,  and  shade. 


ART  IN  COLOUR  PHOTOGRAPHY  245 


Hue. — This  may  be  defined  as  an  extremely  narrow 
portion  of  the  spectrum  which  corresponds  to  a definite 
wave-length,  in  other  words,  it  corresponds  to  a 
certain  definite  colour.  It  corresponds  to  the  pitch  of 
a musical  note.  In  a wider  sense,  it  comprises  a mix- 
ture of  any  pair  of  primaries  in  any  proportion. 
Painters  employ  it  in  this  sense. 

Tint. — This  is  a hue  diluted  with  white.  The 
amount  of  admixture  of  white  defines  the  tint.  It 
corresponds  to  quality  in  the  case  of  musical  sounds. 
The  addition  of  white  will  alter  the  tint  without  affect- 
ing the  hue. 

Shade. — This  differs  from  a tint  in  that  the  hue  is 
altered  by  the  addition  of  black,  or,  indirectly,  by  vary- 
ing the  illumination.  It  corresponds  to  loudness  when 
referring  to  musical  sounds.  For  example,  the  hue 
“ red  ” gives  every  variation  of  tint  from  red  to  white, 
and  every  variation  of  shade  from  red  to  black. 
Hence,  a tint  is  any  colour  to  which  white  has  been 
added,  a shade  any  colour  from  which  white  has  been 
subtracted. 

§ 116.  How  one  can  shade  or  produce  Shadows 
in  a Coloured  Drawing  or  Photographic  Print  in 
Colours. — It  is  a common  error  with  a beginner  to 
imagine,  if  he  wishes  to  make  a coloured  painting  or 
print  look  darker  or  in  shadow,  he  must  add  more 
colour  and  make  it  thicker.  Making  the  colour  thicker 
only  makes  it  deeper  in  tint  and  more  saturated;  it 
never  makes  it  darker  or  more  in  shadow.  If  he  wants 
to  make  a colour  darker,  i.e.  to  place  it  in  shadow, 
he  must  make  it  greyer.  This  can  be  done,  not  toy 
adding  black  paint , however  diluted  with  water,  but  by 


246  PHOTOGRAPHY  IN  COLOURS 


painting  over  the  surface  with  a mixture  of  the  three 
primaries,  in  other  words,  a mixture  of  red,  yellow,  and 
blue  paints  in  certain  proportions,  the  proportions 
depending  on  the  degree  of  grey  or  shade  required. 
As  a rule,  much  yellow,  a little  red,  and  more  blue  is 
required,  and  if  one  does  this  to  any  water-colour 
drawing,  it  will  have  the  effect  of  being  in  shade.  The 
best  way  to  throw  a light-yellow  surface  into  shadow 
is  to  add  a small  quantity  of  red  while  the  paper  is 
still  wet.  This  will  give  it  an  orange  colour.  Then 
add  somewhat  more  blue  paint.  In  the  same  way, 
in  order  to  darken  a blue  surface,  add  some  red,  and 
then  a good  deal  more  yellow.  For  a red  surface, 
wash  over  with  a little  blue,  and  then  considerably 
more  yellow.  By  varying  the  proportions  of  these 
three  colours,  you  can  obtain  any  shade  you  please. 
In  each  case  the  colours  must  be  mixed  on  the  paper. 
Instead  of  spreading  the  two-shade  colours  on  with  a 
brush,  it  often  greatly  improves  the  effect  if  the  colours 
be  finely  stippled  over.  The  dots  are  quite  invisible  at 
a very  short  distance,  and  the  colours  blend  together, 
and,  if  well  done,  the  effect  is  often  most  charming, 
giving  rise  to  a very  soft  and  delicate  surface.  Useful 
pigments  for  this  purpose  are 1 — 


Yellows. 

Chrome. 

Gamboge. 

Ochre. 

Raw  sienna. 


Reds. 

Burnt  sienna. 
Brown  madder. 
Crimson  lake. 
Vermilion. 
Light  red. 


Blues. 

Cobalt  blue. 
French  blue. 
Prussian  blue, 
Ultramarine. 


1 These  colours  are  taken  from  Mr.  H.  A.  Rankin’s  admirable 
little  book  entitled  “ The  Teaching  of  Colour,”  published  by  Sir 
Isaac  Pitman  and  Sons. 


ART  IN  COLOUR  PHOTOGRAPHY  247 


§ 117.  General  Hints  as  to  Colour. — A few  of 

the  following  hints  may  prove  useful  to  the  beginner 
in  relation  to  this  subject. 

Always  avoid  very  large  areas  of  the  same  colour 
when  taking  a coloured  photograph,  as  they  weary  the 
eye  and  detract  the  attention  from  the  general  compo- 
sition. The  more  brilliant  and  prominent  the  hue,  the 
smaller  it  should  be  in  the  case  of  a coloured  'print . 
But  in  the  case  of  a transparency , very  much  larger 
areas  may  be  employed  with  effect,  owing  to  the  light 
shining  through  the  positive,  and  in  this  case  the 
colours  cannot  be  too  brilliant. 

Of  all  the  colours,  red,  and  especially  a bright 
scarlet  or  vermilion,  is  the  one  which  requires  the 
greatest  judgment  in  photographing.  It  is  surprising 
how  little  red  or  orange  there  is  in  Nature,  and  especi- 
ally in  landscapes,  if  we  except  the  red  skies  at  sunset ; 
and  when  it  does  occur,  it  is  almost  invariably  toned 
down  by  a liberal  admixture  of  browns,  greys,  and 
other  sombre  colours.  I tested  this  by  taking  a large 
number  of  photographs  through  a spectrum-blue  glass, 
which  cut  off  all  the  red  and  orange  rays,  leaving  the 
yellow,  for  the  most  part,  unaffected.  And  I found,  to 
my  astonishment,  that  many  of  the  prints  showed  very 
little  difference  from  those  taken  on  a panchromatic  plate 
through  a yellow  screen,  which,  of  course,  is  sensitive 
to  all  colours,  including  red  and  orange.  Still,  in  almost 
every  colour  print,  and  certainly  in  every  transparency, 
one  or  two  large  patches  of  bright  red,  together  with 
others  of  a more  sombre  shade,  greatly  improved  the 
effect. 

It  is  well  to  remember  that  both  bright  red  and 


248  PHOTOGRAPHY  IN  COLOURS 

orange  have  a stimulating  effect  on  the  senses  and 
exhilarate;  bright  sky-blues,  nearly  saturated,  give  a 
pleasing  effect.  On  the  other  hand,  very  pale  blues 
and  light  greys  are  anything  but  pleasing  over  large 
areas,  while  greens  have  a soothing  effect  and  are 
especially  restful  to  the  eyes. 

Large  areas  of  white  fatigue  the  senses  and  dazzle 
the  eyes,  whereas  small  areas  have  the  opposite  effect. 
As  we  have  just  stated,  the  amount  of  unmixed  red  in 
landscapes  is  so  small  that  it  is  often  advisable  to 
shift  the  point  of  view  so  as  to  include  if  possible  some 
considerable  amount  of  red  or  other  bright  colour  in 
the  foreground  or  middle  distance,  in  order  to  brighten 
up  the  picture.  This  is  especially  the  case  if  the  view 
is  very  largely  made  up  of  green  foliage. 

In  order  to  make  a pleasing  picture,  it  is  most 
important  to  see  that  whatever  colour  is  dominant — 
by  which  we  mean  that  it  occupies  the  greater  part  of 
the  picture— the  colour  should  not  be  of  the  same 
uniform  hue  or  shade,  but  that  it  should  be  repeated, 
or  echoed,  as  it  were,  in  various  shades  and  tints 
throughout  the  picture,  so  as  to  give  a sense  of  repose, 
while  at  the  same  time  the  main  subject  of  the  picture 
should  have  the  most  pronounced  hue  of  all,  so  as  to 
fix  the  interest  on  that  spot.  Every  picture  should 
exhibit  unity  of  purpose  throughout,  as  well  as  one 
and  only  one  idea  and  episode,  all  the  other  parts 
being  contributory  and  accessory  to  it.  The  com- 
position of  a picture  is  every  whit  as  important  in 
colour  photography  as  in  an  oil  or  watercolour  paint- 
ing. No  one  will  dispute  the  fact  that  a photograph  of 
a house  or  cottage  will  have  a more  pleasing  effect  if 


ART  IN  COLOUR  PHOTOGRAPHY  249 


it  occupies  only  a portion  of  the  picture  instead  of  the 
whole  width,  and  that  the  house  should  be  balanced 
by  setting  it  off  with  a certain  amount  of  foreground. 
Again,  it  will  be  far  more  artistic  if  the  building  be 
photographed  from  one  side,  so  as  to  represent  one  of 
the  side  walls  in  addition  to  the  front,  instead  of  being 
taken  horizontally  so  as  to  show  merely  the  front  of 
the  house  without  any  depth  or  sense  of  perspective 
whatever.  No  one  with  any  artistic  feeling  will  take 
a photograph  of  a road  which  stretches  vertically  up 
through  the  centre  of  the  picture,  nor  will  he  set  his 
camera  right  in  the  middle  of  the  road,  with  the  latter 
reaching  nearly  up  to  the  lens,  so  that  the  print  will 
exhibit  a vast  isosceles  triangle  of  bare  road  cutting 
out  the  greater  part  of  the  view.  He  would  certainly 
improve  his  picture  if  he  raised  his  camera  as  high 
above  the  road  as  possible.  By  this  means  the  nearest 
distance  which  forms  the  immediate  foreground  of  the 
road  can  be  photographed  a very  considerable  distance 
away  from  the  camera,  so  that  the  sides  of  the  road 
would  appear  more  nearly  parallel,  and  thus  produce 
a less  violent  perspective. 

In  order  to  contribute  to  depth,  it  is  well  to  arrange 
the  main  lines  of  the  picture  obliquely,  or  more  or  less 
in  diagonals  to  the  sides.  This  will  allow  of  a con- 
vergence of  the  perspective  lines  towards  the  vanishing 
point,  as  well  as  gradations  in  size  of  similar  objects 
as  they  appear  to  recede,  and  this  will  largely  add  to 
the  sense  of  depth.  This  is  always  an  important 
element  in  a picture,  because,  in  Nature,  every  object 
possesses  three  dimensions  which  in  consequence 
give  rise  to  a solid  stereoscopic  effect ; whereas  a 


250  PHOTOGRAPHY  IN  COLOURS 


picture  has  of  necessity  only  two  dimensions,  viz. 
height  and  breadth,  and  to  give  the  sense  of  three 
dimensious,  the  artist  has  to  create  a number  of 
illusions  and  contrivances.  To  this  end  he  employs 
shadows,  aerial  effects,  vanishing  lines,  contributory 
curves,  and  various  other  devices  for  increasing  the 
sense  of  perspective  and  depth.  Moreover,  in  colour 
photography,  any  colour  can  be  intensified  or  reduced 
by  local  intensification  or  reduction,  or  the  whole  can 
be  modified  so  as  largely  to  contribute  to  these  qualities. 

§ 118.  Shadows. — Shadows  are  especially  useful  in 
order  to  give  depth  and  plasticity  (stereoscopic  effect)  to 
the  picture,  and  so  remove  the  appearance  of  flatness 
which  is  so  conspicuous  a fault  in  most  of  the  photo- 
graphs made  by  beginners.  This  fault  may  be  avoided 
— if  there  is  no  choice  in  taking  the  view — in  a very 
simple  manner  by  merely  altering  the  position  of  the 
source  of  illumination,  or  if — as  in  landscape  work — 
that  is  impossible,  one  must  alter  the  direction  of  the 
view,  so  as  to  get  the  source  of  light  more  on  one  side. 
If  that  is  of  no  use,  one  must  wait  until  the  sun  occupies 
another  position,  or  else  defer  taking  the  picture  until 
the  evening,  when  the  shadows  will  be  longer,  and  the 
high  lights  have  diminished.  The  position  of  the  sun 
is  of  paramount  importance,  and  its  effect  on  the 
picture  should  be  a matter  of  careful  study  if  you 
really  wish  to  excel  in  your  work. 

Never  photograph  any  one  in  a white  dress,  or 
indeed,  a large,  white  object  of  any  kind  if  it  be 
illuminated  by  a light  directly  in  front  of  it,  i.e.  behind 
the  camera.  Always  secure  a side  light,  or  even  one 
well  above  and  in  front  of  you,  if  the  sun  be  sufficiently 


ART  IN  COLOUR  PHOTOGRAPHY  25  I 


high  up  to  allow  of  the  lens  being  cast  into  shadow. 
By  this  means  you  will  be  able  to  get  as  many  half- 
tones as  possible.  White,  unless  in  comparatively 
small  quantities,  is  very  unsatisfactory.  In  ordinary 
monochrome  photography,  it  is  easy  enough  to  re- 
produce pure  white  over  a surface  of  any  size,  but  in 
colour  photography  it  is  almost  impossible  to  secure 
a large,  unbroken  surface  of  pure  white  by  combining 
the  three  primary  colours.  If,  however,  the  surface  is 
small,  or  largely  broken  up  by  half-tones  and  shadows, 
very  pure  whites  may  be  reproduced  and  remarkably 
pleasing  effects  obtained.  Beware  of  going  to  the  other 
extreme,  and  allow  black  shadows  to  fall  on  the  figure. 
This  is  especially  important  when  photographing  the 
face  and  other  exposed  parts. 

Never  take  snow  scenes  with  the  sun  directly  behind, 
or  you  will  get  a flared  picture  with  no  half-tones, 
whereas,  if  the  sun  is  nearly  or  even  quite  in  front  of 
the  camera  and  fairly  high  up,  the  results  are  often 
magnificent,  the  snow  and  ice  being  full  of  lovely 
purple  half-tones.  But  to  secure  this  effect  you  must 
screen  the  lens  from  direct  sunlight  by  means  of  a 
flap-shutter,  or  your  hand  or  hat,  or  otherwise  you  will 
inevitably  spoil  the  picture  by  the  light  striking  the 
lens  obliquely  and  causing  flare  and  fog. 

§ 119.  Choose  Simple  Subjects. — The  beginner 
is  generally  impressed  by  the  beauty  of  a distant 
panoramic  view,  and  naturally  concludes  that  a colour 
photograph  of  what  is  spread  out  before  him  will 
make  a most  impressive  picture.  When  he  has  taken 
and  finished  the  picture,  he  is  invariably  disappointed 
with  the  effect  produced.  The  reason  for  this  is  two- 


252 


PHOTOGRAPHY  IN  COLOURS 


fold.  In  the  first  place,  the  view  which  so  enchanted 
him  embraced  an  angle  of  about  150  degrees  or  more, 
whereas  the  actual  angle  of  the  picture  taken  only 
includes  about  40  or  at  most  45  degrees ; in  fact,  a 
mere  slice  of  the  panorama.  No  wonder  that  it  proved 
disappointing.  Again,  unless  a very  large  plate  be 
used,  everything  in  the  far  distance  appears  dwarfed, 
and  all  detail  is  lost,  so  that  the  eye  wanders  aimlessly 
over  the  picture  without  anything  large  enough  for  the 
eye  to  dwell  upon  with  any  satisfaction,  since,  unlike 
an  ordinary  photograph,  it  can  only  be  enlarged  with 
difficulty,  and  not  at  all  if  taken  on  a single  colour- 
plate.  The  experienced  photographer,  on  the  other 
hand,  will  select  a far  more  modest  subject,  often  one 
which  to  the  casual  observer  would  appear  entirely 
destitute  of  interest ; such,  for  example,  as  a dripping 
well,  set  off  with  a group  of  ferns,  or  even  an  old 
rustic  porch,  or  perhaps  a bank  covered  with  primroses. 
Such  subjects  would  seem  very  commonplace,  and  he 
would  certainly  never  dream  of  photographing  them  ; 
and  yet  they  are  the  subjects  which  are  most  frequently 
selected  for  a medal.  Wherever  possible,  introduce 
figures,  and  see  that  they  wear  ~bright  colours,  and  never 
black.  Life  of  some  kind,  whether  animals  or  human 
beings,  always  make  a picture  more  interesting ; they 
balance  the  surroundings,  throw  back  the  distance, 
and  give  strength  and  plasticity  to  the  whole.  But 
when  you  introduce  figures  into  the  landscape,  don’t 
on  the  one  hand  commit  the  fault  of  making  the 
figures  so  small  as  to  be  lost  in  the  landscape,  nor,  on 
the  other  hand,  of  placing  the  figures  right  in  the  fore- 
ground so  as  to  reduce  the  landscape  to  a mere  back- 


ART  IN  COLOUR  PHOTOGRAPHY  253 


ground  of  the  figure.  Of  the  two,  this  is  by  far  the 
worst  fault.  In  a landscape  the  figures  should  always 
be  selected  to  harmonize  with,  and  balance  the  picture. 
An  inspection  of  Mr.  H.  P.  Robinson’s  picture  entitled, 
“ Wayside  Gossip,”  or  of  Gale’s  “ Sleepy  Hollow,”  re- 
produced in  the  former’s  little  handbook  of  “ Art 
Photography,”  1 will  show  what  is  meant. 

It  is  also  important  to  see  that  the  colour  of  the 
dress  harmonises  with  its  immediate  surroundings. 
Thus,  a red  parasol  or  cloak  will  often  work  wonders 
in  brightening  up  a landscape.  You  must  be  sure  and 
see  that  there  is  sufficient  contrast  to  bring  the  figure 
into  relief,  so  as  to  catch  the  eye  at  once.  If,  there- 
fore, the  person  is  wearing  dark  clothes,  do  not  place 
him  in  the  shadow  or  in  front  of  dark  foliage,  or  the 
broad  trunk  of  a tree,  but,  rather,  select  a position  in 
which  the  surroundings  are  as  light  as  possible.  A 
bright  object  placed  next  to  a dark  object  or  a deep 
shadow  will  make  the  bright  one  appear  still  brighter 
and  the  dark  object  darker.  The  more  contrast  the 
greater  the  prominence  and  the  depth.  Try  and  imi- 
tate Turner,  Claude,  or  Cuyp,  and  employ  every  artifice 
to  give  the  effect  of  depth  and  plasticity  to  your  pic- 
ture. A visit  to  the  Turner  room  in  the  National 
Gallery  will  well  repay  a visit.  Turner  was  in  the 
habit  of  selecting  some  point  such  as  the  sun,  which 
he  placed  above  the  horizon  as  the  point  of  fixation,  so 
as  to  give  the  impression  of  infinite  distance,  aud  all 
the  objects  in  the  foreground  and  middle  distance  he 
artfully  arranged  so  as  to  direct  the  observer’s  eye 
towards  this  point. 

1 See  “ The  Amateur  Photographer’s  Library,”  No.  4.  Pub- 
lished by  Hazel,  Watson,  & Yiney,  Ltd.,  London. 


254 


PHOTOGRAPHY  IN  COLOURS 


One  of  the  great  secrets  of  pictorial  photography  is 
to  endeavour  to  compensate  and  balance  all  leading 
lines  and  masses.  If  the  lines  incline  in  one  direction, 
try  and  arrange  the  point  of  view  so  that  other  lines 
compensate  by  their  inclination  in  a nearly  opposite 
direction.  By  this  means  a sense  of  stability  is  secured, 
and  the  observer  becomes  unconsciously  satisfied. 

§ 120.  Portrait  Photography. — When  taking  a 
group,  endeavour  to  arrange  the  figures  so  that  they 
stand  or  sit  down  naturally,  just  as  they  would  do  if  you 
happened  to  come  upon  the  group  unawares.  If  some 
of  the  persons  are  standing  with  their  backs  to  the 
camera,  or  are  sitting  down,  or  happen  to  have  their  faces 
turned  round  in  another  direction,  so  much  the  better, 
as  it  will  appear  more  natural.  Very  often  the  effect 
will  be  greatly  enhanced  by  arranging  the  figures  in 
an  irregular,  pyramidal  group,  with  the  heads  at  dif- 
ferent heights,  and  the  faces  turned  towards  each  other 
in  a natural  manner,  as  if  in  conversation.  This  idea 
is  carried  out  to  perfection  in  H.  P.  Robinson’s  cele- 
brated photograph,  “ A Merry  Tale,”  which  will  be 
found  reproduced  in  the  book  referred  to  on  a previous 
page.  His  work  on  “ Picture  Making  by  Photography” 
contains  many  illustrations  which  bear  out  in  a 
graphic  way  the  maxims  I have  laid  down  in  this 
chapter. 

Most  amateurs,  and  even  many  professional  photo- 
graphers, are  in  the  habit  of  arranging  a group  as  if 
the  members  were  drawn  up  on  parade,  with  the 
result  that  there  is  no  picture  at  all.  One  sees  merely 
a row  of  individuals,  all  staring  straight  in  front  of 
them.  This  is  a fatal  mistake,  and  the  further  it  is 


ART  IN  COLOUR  PHOTOGRAPHY  255 

departed  from  the  nearer  it  will  be  to  an  artistic 
picture.  Often  a portrait  taken  with  the  back  turned 
round  so  that  the  face  is  invisible  in  the  camera,  will 
actually  be  a better  portrait,  and  more  characteristic  of 
the  person,  than  a full-faced  one.  One  of  Whistler’s 
greatest  triumphs — a portrait  of  his  mother — was  taken 
in  this  way. 

Let  us  take  for  example  a photograph  of  a couple  of 
children.  The  photographer  who  has  no  idea  of  art 
will  place  them  side  by  side  right  in  front  of  the 
camera,  and  staring  into  it  like  a couple  of  dolls,  or 
else  leaning  their  heads  together  in  an  idiotic  fashion, 
while  he  sets  the  figures  off  by  a hideously  painted 
background,  or,  worse  still,  by  a wall-paper  decorated 
with  endless  baskets  of  impossible  flowers.  On  the 
other  hand,  the  artist  will  go  to  work  very  differently. 
He  will  think  out  some  natural  scene  out-of-doors. 
Perhaps  he  will  arrange  the  children  playing  at  hide- 
and-seek  on  each  side  of  a moss-covered  trunk  of  a big 
tree,  and  just  catch  the  expression  of  the  one  looking 
round  the  corner  with  the  face  in  full  view  and  laugh- 
ing merrily,  while  the  other  has  its  face  turned  partly 
away,  and  is  peering  round  the  opposite  side  of  the 
tree.  Or  he  may  photograph  them  unawares,  sitting 
down  on  a bank  or  under  a hedge  gathering  black- 
berries or  buttercups,  which  they  are  busy  placing  in 
a basket.  What  could  be  more  charming  than  the 
picture  of  two  children  by  Millais,  entitled,  “ Cuckoo  ”? 
This  is  art,  because  it  illustrates  a scene  in  their  real 
lives.  They  are  doing  something  which  they  are 
accustomed  to  do,  in  a natural  manner  and  without 
any  pose.  All  that  is  required  in  either  case  is  to 


256  PHOTOGRAPHY  IN  COLOURS 

arrange  the  dress  surroundings  and  illumination  so  as 
to  harmonise  perfectly,  and  quietly  await  the  oppor- 
tunity when  the  children  are  unconscious  that  their 
portraits  are  being  taken,  and  as  a result  you  will  have 
a real  picture.  I recollect  seeing  in  one  of  the  photo- 
graphic exhibitions  in  London  a photograph  of  a 
number  of  fishermen  and  boys  leaning  over  a sea-wall 
with  some  shipping  in  the  background.  It  was  called 
“ A Stern  View,”  and  hardly  a single  face  could  be 
seen.  But  this  picture  was  wonderfully  true  to  life, 
and  was  so  excellent  that  I believe  it  was  awarded  a 
medal.  It  cannot  be  too  often  repeated  that  unless 
you  mix  brains  with  your  colours,  whether  in  a paint- 
ing or  a photograph,  it  will  fail  to  have  much  value. 
Although  a photograph  is  incapable  of  producing  the 
full  individuality  of  the  artist,  nevertheless  a colour 
photograph  will  always  contain  evidence  of  the  photo- 
grapher’s soul  and  inspiration  in  every  part  of  the 
picture,  if  it  is  to  rank  as  a really  artistic  production. 

§ 121.  Backgrounds. — A suitable  background  greatly 
adds  to  the  beauty  of  any  coloured  object  which  may  be 
selected  for  reproduction.  Pottery,  flowers,  fruit,  fish, 
butterflies,  and  other  natural  objects  will  often  appear 
tame  and  commonplace  without  a background,  or  with 
an  unsuitable  one,  but  will  be  made  beautiful  and 
striking  when  shown  up  by  an  artistic  background. 
If  the  object  requires  a strong  relief,  a very  dark  twill, 
or,  even  better,  a piece  of  black  velvet  or  velveteen, 
will  do  admirably,  provided  the  object  be  light  in 
colour,  and  especially  if  it  is  a bright  yellow  or  orange. 
On  the  other  hand,  a dark  object,  or  one  showing  a 
fully  saturated  colour,  such  as  crimson,  emerald  green, 


ART  IN  COLOUR  PHOTOGRAPHY  257 

or  Prussian  blue,  will  show  up  best  against  a creamy 
or  bluish-grey  background.  It  is  rarely  advisable  to 
select  the  exact  opposite  or  complementary  colour,  but, 
as  we  have  already  mentioned,  a hue  some  20  or  30 
degrees  away  from  the  opposite  side  of  the  chromatic 
circle  of  colours  (see  Fig.  28)  may  with  advantage  be 
chosen.  The  complementary  colour  generally  affords 
too  severe  a contrast  to  blend  harmoniously  with  the 
subject. 

Sometimes  the  same  hue  in  a much  darker  or  lighter 
shade  is  pleasing.  Often  greys,  browns,  or  some  other 
neutral  or  composite  hue  will  serve  the  purpose  best. 
As  a rule,  a simple  colour  without  any  pattern  should 
be  chosen.  The  reason  for  this  is  that  any  pattern, 
however  inconspicuous,  will,  to  some  degree,  draw  the 
observer’s  attention  away  from  the  subject  of  the  pic- 
ture. It  is,  however,  quite  permissible  to  vignette  the 
background,  or  vary  the  tint  or  shade  over  certain 
parts.  In  many  cases,  should  strong  relief  be  desired, 
the  light  should  be  so  arranged  that  the  object  will 
cast  a deep  shadow  over  a portion  of  the  background. 
This,  of  course,  can  be  readily  effected  by  arranging 
the  lighting  so  that  it  emanates  from  one  side  only, 
the  rest  of  the  room  being  more  or  less  in  subdued 
light. 

The  backgrounds  which  I can  recommend  from 
experience  are  brown,  light-brown,  pale-green,  dark- 
green,  and  dark-blue  twill,  or  stiff  linen  cloth,  which 
should  unroll  quite  flat,  without  any  creases.  Also 
red,  purple,  and  cream-coloured  velvet  or  velveteen 
are  most  useful  for  exhibiting  many  brilliant  objects 
such  as  jewels,  coins,  butterflies,  and  beetles,  etc. 

s 


258  PHOTOGRAPHY  IN  COLOURS 

These  show  up  splendidly  by  contrast  on  cream  or 
coloured  velvet,  and  they  can  be  made  to  appear  in 
high  relief  by  shutting  out  all  superfluous  light  and 
confining  the  illumination  to  one  side  of  the  object,  so 
as  to  cast  deep  shadows. 


APPENDIX 

Theories  of  Colour  Vision. 

Several  theories  have  been  made  to  explain  the 
phenomena  connected  with  colour  vision.  But  none 
of  them  will  explain  all  the  facts,  although  each  of 
them  in  turn  has  its  special  advantages.  The  two 
theories  most  in  favour  are  those  known  as  the  Young- 
Helmholtz  and  the  Hering  theories. 

1.  Young -Helmholtz  theory . — This  theory,  which  was 
originally  suggested  by  Thomas  Young  about  the  year 
1807,  and  slightly  modified  by  Helmholtz,  assumes 
that  there  are  three  types  of  nerves  in  the  retina,  each 
tuned  to  respond  to  one  of  the  three  primary  colour 
sensations,  viz. — red,  green,  and  blue- violet.  By 
decomposition  of  the  three  photo-chemical  substances 
stored  up  in  the  retina,  the  nerve  fibres  are  stimulated 
to  respond  to  the  frequencies  of  vibration  corresponding 
to  these  colours.  These  vibrations  generate  impulses 
in  the  nerve  ends  which  are  conveyed  to  the  visual 
centres  in  the  grey  matter  of  the  brain,  and  the  mind 
perceives  the  colours  developed. 

By  suitable  mixing  of  these  three  colours,  every 
shade  and  hue  can  be  produced.  Thus  White  is  the 
result  of  the  fusion  of  all  three  colour  sensations,  or  of 
any  two  complementary  coloured  lights,  while  Black 


26o 


PHOTOGRAPHY  IN  COLOURS 


on  the  other  hand  results,  from  the  absence  of  all 
stimulation  of  those  parts  which  are  capable  of 
responding  to  colour  stimuli. 

Helmholtz  constructed  a scheme  to  illustrate  the 
effect  of  stimulating  the  photo-chemical  substances 
which  produce  the  three  colours  in  different  degrees 
according  to  the  different  colour  observed.  Thus 
yellow  will  be  produced  by  the  fusion  of  much  red 
and  green,  together  with  a trace  of  blue ; while  blue  is 
caused  by  the  full  stimulus  of  blue  substance  with  a 


Fig.  30. — Scheme  illustrating  the  Young-Helmholtz  Theory  of 
Colour  Vision. 

The  curves  represent  the  intensity  of  stimulation  of  the 
three  colour  substances. 

little  green  and  a mere  trace  of  red.  It  will  be  noticed 
from  the  annexed  figure  (Fig.  30)  that  it  is  impossible 
to  stimulate  any  one  of  the  primaries  without  at  the 
same  time  affecting  to  some  extent  the  other  two. 

Unfortunately,  if  we  try  to  imitate  in  practice  any 
of  the  colour  sensations  in  the  same  proportions  as  the 
curves  in  Helmholtz’s  diagram,  an  almost  colourless 
or  dirty  white  will  result.  Hence  some  physiologists 
have  suggested  that  the  curves  should  take  a different 
form. 

There  are  many  objections  to  the  Young-Helmholtz 


APPENDIX 


261 


theory  in  addition  to  those  mentioned  in  § 22.  Thus 
we  are  not  conscious  that  the  sensation  of  white  is  a 
blend  of  two  or  more  colours,  as  we  invariably  are  in 
such  mixtures  as  peacock  green  or  purple.  Again, 
towards  the  periphery  of  the  retina  we  can  perceive 
whites  and  greys  notwithstanding  that  part  is  colour 
blind.  Moreover,  we  perceive  black  as  a real  impres- 
sion, although  Helmholtz  explained  it  as  being  due  to 
a state  of  quiescence  or  rest  of  the  visual  cells. 

2.  Hering’s  theory. — This  theory  also  assumes  three 
photometric  substances  which  give  rise  to  six  different 
qualities  of  sensation,  arranged  in  three  pairs,  one 
sensation  in  each  pair  undergoing  assimilation,  while 
its  fellow  undergoes  disassimilation.  Thus  we  have  a 
white-black  substance,  which  when  acted  upon  by 
light  undergoes  disassimilation,  and  gives  rise  to  the 
sensation  of  white,  while  the  same  substance  becoming 
assimilated  gives  rise  to  a black  sensation.  In  the 
same  way  a red-green  substance  and  a yellow-blue 
substance  exist  in  the  retina,  each  of  which  by  assimi- 
lation or  disassimilation  results  in  the  sensation  of  one 
of  its  components.  These  six  sensations  can  be 
tabulated  as  follows  : — 


Photochemical  substance. 
Red-green 

Yellow-blue  . 

White-black  . 


Retinal  process.  Sensation, 
f Disassimilation  = Red. 

( Assimilation  = Green. 

| Disassimilation  = Yellow. 
(Assimilation  = Blue. 
(■Disassimilation  = White. 
(Assimilation  = Black. 


This  theory  gives  a definite  objective  cause  for  the 
sensation  of  white,  black,  and  yellow,  and  in  this 
respect  is  superior  to  the  Young-Helmholtz  theory.  It 


262 


PHOTOGRAPHY  IN  COLOURS 


also  accounts  for  yellow  as  a distinct  sensation  which 
the  physiologists  demand.  Moreover,  it  is  in  harmony 
with  the  fact  that  in  certain  birds  and  reptiles  we  find 
yellow  as  well  as  red  oil  globules  in  the  bacillary 
layer,  and  also  in  the  majority  of  tapeta  we  find  not 
only  red  and  green  but  also  intense  yellow  colours  over 
large  areas.  Instead  of  complementary  colours  red  and 
green  should  be  termed  antagonistic  colours.  When 
they  act  simultaneously  on  a retinal  cone,  the  effects 


R G W Y.B 


Fig.  81. — Scheme  to  illustrate  the  Hering  Theory  of 
Colour  Vision  (after  Foster). 

The  curves  above  the  axis,  xx,  illustrate  catabolic  changes 
(disassimilation),  those  below  the  axis  anabolic  changes 
(assimilation). 

neutralise  one  another  and  the  result  is  white,  or,  as 
Heron  would  say,  the  disassimilation  effect  remains 
over  which  produces  white.  It  will  be  seen  from  the 
above  description  that  the  Young-Helmholtz  theory 
agrees  best  with  physical,  while  Hering’s  theory  agrees 
best  with  physiological,  phenomena. 


APPENDIX 


263 

1.  Table  of  Exposures  for  Separate  and 
Combined  Colour  Plates 

The  following  table  giving  approximately  the  correct 
exposures  for  the  Autochrome,  Omnicolore,  and  Paget 
plates,  has  been  revised  by  Messrs.  Lumiere,  Messrs. 
Jougla,  and  Mr.  Dawson  respectively,  to  which  the 
Dufay  has  been  added.  The  ratio  of  the  five  plates 
with  their  proper  filters  is  as  follows  in  seconds  (")  or 
in  minutes  ('). 

Autochrome  1",  Dufay  f",1  Omnicolore  §",  Paget 
separate  plate  \ to  or  taking  the  Paget  separate 
plate  as  unity,  we  get — 

Paget  separate  1",  Omnicolore  5",  Dufay  4",  Auto- 
chrome 3 to  4".  The  following  table  gives  the  ex- 
posures between  the  middle  of  May  and  middle  of 
August,  with  lens  working  at  E/8  and  time  of  day  10.30 
to  2.30.  Bright  sky,  white  clouds  or  sun.  In  cloudy 
weather  increase  exposure  3 to  6 times. 

For  the  benefit  of  those  who  do  not  understand 
ratio-apertures,  the  following  table  will  be  found  useful. 
If  we  assume  the  exposure  with  a stop  of  F/8  = 1 sec., 
then — 


F/4' 

requires 

i 

sec, 

F/5,  6 

99 

1 

2 

11 

F/6,3 

11 

s 

If 

F/6,8 

ff 

3 

■i 

„ 

F/8 

„ 

1 

n 

F/ll 

91 

2 

M 

F/16 

11 

4 

91 

F/22 

91 

8 

,, 

F/32 

„ 16 

91 

1 According  to  the  Author’s  experience  Dufay  plates  should 
have  11"  and  Omnicolore  1£",  i.e.  slightly  longer  exposure  than 
the  Autochrome. 


264  PHOTOGRAPHY  IN  COLOURS 


Bright  sunshine  10.30-2.30,  May  15-Aug.  15,  F/8. 


Subject. 

Omni- 
col  ore.1 

Dufay.1 

Auto- 

chrome. 

Paget. 

Portrait  or  flower 

study,  well  lighted 
room  near  window 
with  white  reflector 

30"-60" 

25"-50" 

20"-40" 

7"-15" 

Portraits,  flowers, 

fruit  studies,  studio 
well  lighted  . 

Ditto,  ordinary  room 

15"-25" 

12"-20" 

10"-16" 

3"-5" 

not  near  window  . 
Ditto,  ditto,  in  open 

4'-6' 

3^-5' 

3|'-5' 

1-2' 

air,  bright  sunshine 
Stained-glass  window 

4"-6" 

3i"-5" 

3i"-5" 

l'-2' 

north  aspect  with 
much  ruby  glass. 

45"-80" 

35"-60" 

27"-48" 

9"-16" 

With  addition  of  K1 
filter  to  the  lens 

3-5' 

2'  30"-4'  30" 

2'-3'  30" 

40"-80" 

Open  landscape,  no 
heavy  objects  in  fore- 
ground, well  lighted 
Ditto,  strong  fore- 

l"-2" 

r-ir 

a'Lii” 

5 ±4 

in  2," 
5 5 

ground  .... 
Ditto,  very  heavy 
foreground  . 

3i"-7" 

3"-6" 

2±"-5" 

l"-2" 

10"-20" 

9"-18" 

6"-12" 

2"-4" 

Eiver  view,  water  in 

foreground,  no  dark 
objects,  well  lighted 
Open  lake  or  sea  view, 
sun  or  bright  light 

r- 1" 

3"_3" 
B 4 

r-r 

10  5 

on  water 

No  large  objects  near, 
with  addition  of  K1 

r-r 

1"_2" 
5 5 

r-r 

1 II  1 II 
20  10 

filter  

2r-4r 

2"-4" 

ir-sr 

r-r 

Kr1",, 

Snow  and  ice  scene  . 

l"~¥' 

1"_2" 

WO  “15 

No  dark  rocks,  well 

5 5 

lighted,  with  K1 
filter  added  . 

2"-4" 

lf"-3i" 

li"-2§" 

f "-f" 

Ditto,  ditto,  in  win- 

ter, much  snow,  and 
K1  filter  added  . 

4"-8'.' 

3|"-7" 

2§"-5|" 

l"-2" 

1 According  to  the  Author’s  experience,  the  exposure  of  the 
Dufay  and  Omnicolore  plates  should  be,  if  anything,  longer  than 
that  of  the  Autochrome.  If  the  exposure  of  the  Autochrome 
be  taken  as  1",  that  of  the  Dufay  should  be  about  1|",  and 
the  Omnicolore  1£".  The  exposure  of  a Paget  plate  is  about 
one-third  that  of  an  Autochrome. 


APPENDIX 


265 

Stained  glass  windows  with  much  ruby  glass  require 
a very  full  exposure,  or  the  reds  will  appear  brick-red 
and  weak.  All  ice  and  snow  and  seascapes  require  a 
second  filter  to  suppress  the  excess  of  violet  light ; 
Wratten’s  K1  will  do.  It  should  be  fixed  in  front  of 
the  lens  during  exposure.  A K2  filter1  or  a second 
Lumiere  filter  held  in  front  of  the  lens  during  one- 
half  of  the  exposure  is  recommended  by  some  workers. 


2.  Table  op  Exposures  op  Sunsets  for  Auto- 
chromes (F.  Gremier). 


60  minutes  before  scheduled  time  of  sunset  (F/8)  1£  secs. 


45 

„ 

jy 

yy 

3 » 

30 

}) 

yy 

yy 

6 ,, 

15 

M 

yy 

yy 

12  „ 

5 

JJ 

yy 

yy 

22  „ 

At  sunset 

. 30  „ 

5 mins,  past  . 

1 These  filters  can  be  obtained  from  Wratten  and  Wainwright, 
Photographic  Plate  Manufacturers,  Croydon,  England. 


266 


PHOTOGRAPHY  IN  COLOURS 


3.  Table  of  Additive  Colour  Effects,  or  Colour 
Synthesis  [Helmholtz). 


Colour. 

Violet. 

Indigo. 

Cyan- 

blue. 

Blue- 

green. 

Green. 

Greenish- 

yellow. 

Yellow. 

Red 

Purple 

Dark 

Rose 

Light 

Rose 

White 

Whitish- 

yellow 

Golden- 

yellow 

Orange 

Orange 

Dark 

Rose 

Light 

Rose 

White 

Light 

Yellow 

Yellow 

Yellow 

Yellow 

Light 

Rose 

White 

Light 

Green 

Light 

Green 

Greenish- 

yellow 

Greenish- 

yellow 

White 

Light 

Green 

Light 

Green 

Green 

Green 

Light 

Blue 

Sea- 

blue 

Blue- 

green 

Blue- 

green 

Deep 

Blue 

Sea- 

blue 

Cyan-blue 

Indigo 

4.  Table  of  Relative  Brightness  of  a Strongly 
Illuminated  Spectrum  ( Vierordt ), 
Yellow-green  being  considered  as  100. 


Red 2 

Orange 12 

Yellow 78 

Yellow-green 100 

Green 37 

Bine 12*8 

Dark  blue 0*8 

Violet 0-07 


APPENDIX 


267 


5.  Slowest  Exposures  necessary  to  secure 
Sharpness. 

Conditions. — Focal  plane  or  other  highly  effective  shutter. 


Lens,  5 to  6|-in.  focus.  Nearest  object,  50  feet. 

sec.  sec. 

Ordinary  street  scenes  with  traffic.  No  rapid  motion  \ to  jL 
Trees,  moving  with  light  breeze J5  to 


strong  wind 


200  300 


Yachts,  motor  boats,  10  knots  per  hour,  viewed  end  on  to  ^ 

» „ n > ) broadside  on  T^j 

Trains,  80  miles  an  hour,  nearly  end  on,  beyond  50  ft.  gU  to  j-ig 
For  trains  nearly  broadside  on,  motor-cars,  horses  galloping, 
divers,  birds  on  wing,  etc.,  all  calculations  are  useless.  You 
must  use  quickest  shutter,  and  largest  diaphragm  compatible 
with  density  of  negative  and  sharpness  of  image. 

With  F/4  aperture  and  bright  sunlight  in  June  and  July  be- 
tween 11  and  3,  ordinary  street  scenes  beyond  50  ft.  can  be 
taken  with  exposure  on  Paget  (separate)  plates.  If  the  plates 
be  resensitised  by  Grant’s  method,  an  exposure  of  Jq  sec.  can  be 
given  with  F/4  stop. 


6.  Factor  Numbers  ( Watkins). 


Developer. 

Temperature  60°  F.  to  65°  F. 

Soft. 

Factor. 

Normal. 

Hard. 

Adurol 

4 

5 

6 

Amidol 1 

7 

10 

12 

Azol  (Johnson) 

20 

30 

35 

Diogen 

8 

12 

15 

Dionol  (Diamidophenol)  .... 

44 

60 

75 

Edinol 

14 

20 

25 

Eikonogen 

8 

12 

15 

Glycin  (soda) 

6 

8 

10 

,,  (potash) 

9 

12 

16 

1 Amidol  (2  grains  to  the  ounce)  has  according  to  some  writers 
a factor  of  18  for  normal  contrast,  and  Pyro  Metol  14. 

The  “Agfa”  Co.  give  the  following  factors:  Amidol  18, 
Eikonogen  9,  Glycin  10,  Hydroquinone  5,  Pyro-soda  5,  Imogen- 
sulphite  5,  Metol  30,  Metol-hydroquinone  14,  Ortol  10,  Rodinal  30. 


268 


PHOTOGRAPHY  IN  COLOURS 


Developer. 

Factor. 

Temperature  60°  F.  to  65°  F. 

Soft. 

Normal. 

Hard. 

Hydranine 

5 

7 

9 

Hydroquinone 

3 

4-5 

5 

Imogen 

4 

6 

8 

Kachin 

7 

10 

12 

Kodak  powders 

18 

18 

23 

Metol  (Hauff) 

20 

30 

35 

Metol-hydroquinone 

10 

12 

15 

Metaquin 

9 

12 

14 

Ortol 

7 

10 

12 

Paramidophenol 

12 

16 

18 

Paraphenylene 

20 

25 

30 

Pyro-catechin 

7 

10 

12 

„ „ (Crystals)  .... 

22 

30 

35 

Pyro-metol 

6 

9 

11 

Pyro-soda  without  bromide  1 gr. 

13 

18 

22 

>>  j»  >>  2 ,, 

9 

12 

14 

>>  *>  >>  3 ,,  . 

7 

10 

12 

77  77  77  4 7, 

6 

8 

10 

77  77  77  ^77 

Pyro-soda  with  bromide  (half  the 

5 

8 

above  factors) 

— 

— 

— 

Pyro-soda  (Imperial) 

4 

4| 

Quinomet 

9 

12 

14 

Rytol  (Burroughs  Wellcome  & Co.) 

10 

12 

15 

Rodinal 

30 

40 

50 

Synthol 

22 

30 

35 

Note. — The  factor  (at  least  in  the 

case  of  Pyro  and  Amidol) 

varies  inversely  with  the  percentage  amount  of  the  active 


ingredient,  and  inversely  with  the  amount  of  restrainer  (Bromide, 
etc.). 

The  factor  governs  the  contrast  thus : For  more 
contrast,  use  a higher  factor ; for  flat  negative,  or  soft 
contrast,  use  a lower  one. 

Eoughly  speaking,  for  soft  contrasts  use  three-fourths 
of  the  normal  factor ; for  strong  contrasts,  add  one-fifth 
to  the  normal  factor. 


APPENDIX 


269 

Example. — Metol-hydroquinone  is  used  as  the  de- 
veloper. The  image  first  appears  after  20  sec.  Since 
the  factor  is  12,  the  plate  must  be  left  in  the  developer 
for  20  x 12  sec.,  i.e.  4 min.  If  soft  contrast  be  desired, 
the  plate  must  be  left  in  for  20  x 9 sec.  = 3 min.,  and 
for  hard  contrasts,  for  20  x 15  sec.  = 5 min. 

Rule  for  factor  developing. — Multiply  the  number  of 
seconds  that  have  elapsed  between  pouring  on  the 
developer  and  the  first  appearance  of  the  image  by  the 
factor  number.  The  product  gives  the  time  that  the 
plate  should  remain  in  the  developer. 

Double  emulsions,  such  as  Gristoid  films  and 
Thomas’s  plates,  require  at  least  double  the  time  of 
the  factor.  Other  plates,  whether  slow,  fast,  or  iso- 
chromatic,  do  not  appear  to  affect  the  result. 

Rule  for  combination  developers. — If  equal  quantities 
of  each  be  used,  half  the  sum  of  the  two  factors  will 
be  the  factor  of  the  mixture.  If  the  mixture  contains 
unequal  parts,  proceed  as  follows  : — 

Let / = factor  — number  of  solution  A ; 

/'  - factor  — number  of  solution  B ; 
x = number  of  ounces  of  A ; 
y = number  of  ounces  of  B. 

Then  the  combined  factor  number 

F Jv+fy 

x + y 

Example. — A mixture  is  made  of  4 oz.  of  hydro- 
quinone  and  1J  oz.  of  metol.  What  is  the  combined 
factor  F?  The  factor  of  hydroquinone  is  5,  that  of 
metol  is  30,  therefore 

F =-t±fy  = 4 X 5 + (t-5  x 30)  = la  ( ). 

x + y 5-5 


270  PHOTOGRAPHY  IN  COLOURS 


This  does  not  hold  strictly  true  with  pyro  developers, 
which  affect  the  speed  of  other  developers  in  a different 
way. 

As  regards  the  ultimate  image,  all  developers  appear 
to  give  the  same,  or  nearly  the  same,  result,  but  the 
rate  at  which  the  image  first  appears,  as  well  as  the 
time  necessary  to  acquire  a standard  density  and 
gradation,  differ  enormously. 

Thus,  in  the  case  of  rodinal,  metol,  and  dianol 
(diamidophenol)  the  image  flashes  out  quickly,  but  it 
requires  to  be  developed  for  a long  time  in  order  to 
acquire  sufficient  density,  while,  in  the  case  of  strong 
pyro-soda,  adurol,  and  hydroquinone,  the  image  takes 
a long  time  before  appearing,  but  requires  a short 
development  to  secure  the  necessary  density. 

INSTRUCTIONS  FOR  DEVELOPING 
AUTOCHROME  PLATES. 

7.  Pyrogallol  Developer  for  Autochrome 
Plates. 

Some  operators  still  prefer  Lumiere’s  “ Pyro  ” de- 
veloper, which  is  as  follows — 

1st  Development  ( Stock  Solutions). 

A A.  Sod.  Bisulphite  (commercial  solution)  . 2 drops 

Pyrogallol  (dry) 3 grms. 

Bromide  of  Potassium 3 „ 

Distilled  water 100  c.c. 

BB.  Anhydrous  Sodium  Sulphite  ...  10  grms. 

Ammonia  (0*923  or  22  B) 15  c.c. 

Distilled  water 85  c.c. 

For  a half-plate  take  equal  parts  of  AA.  and  BB. 
(10  c.c.  of  each)  and  add  100  c.c.  (3J  ozs.)  of  distilled 
water. 


APPENDIX 


271 


Develop  (if  correctly  exposed)  for  2J  minutes  at  a 
temperature  of  about  60°  F.  This  bath  cannot  be  used 
a second  time. 

2 nd  Development. 

Diamidophenol 0'5  grms. 

Anhydrous  Sodium  Sulphite  . . 1*5  „ 

Distilled  water 100  c.c. 

Leave  positive  in  this  bath  in  full  daylight  from 
3 to  4 minutes  or  more.  Wash  and  allow  to  dry. 


8.  Lumiere’s  Formula  (1908). 


Quantities  sufficient  for 
whole  plate  or  x 6 plate. 

Time  of  action. 

A.— 

Quinomet,  1*5  gms. 

2£  min.  for 

(23  grns.)  1 

correct  ex- 

Water (distilled)  100 

C.C.  (3£  055.) 

Sodium  Sulphite,  10 

posure 

1 Temperature 

grms.  (154  grns.) 

of  baths  15° 

Potassium  Bromide, 

C.  (60°  F.) 

0-6  gm.  (9  grns.) 
Ammonia  (density 
0-923)  3-2  c.c.  (55  m.) 

Remarks. 


Image  should  appear  in 
10  to  14  seconds. 

Factor  = 12. 

Hence  time  of  develop- 
ment = No.  of  secs, 
before  image  first 
appears  multiplied  by 
Factor.  Thus,  if  image 
appears  after  13", 
then  13"  x 12"  = 156" 
or  2|  min.  (approx.). 
Wash  1 min.  in  dark. 


Dissolve  the  Quinomet  in  the  water,  add  the  Sulphite  and 
Bromide,  then  the  Ammonia.1 


AA. — For  use.  To  one  part  of  A add  4 parts  of  water  (preferably 
distilled). 


1 Quinomet  is  another  name  for  Metoquinone,  and  is  a chemical 
compound  and  not  a mechanical  mixture.  It  consists  of  Metol 
in  powder  6-2  gms.,  Hydroquinone  8-8  gms.  It  is  sold  in  a solid 
form  by  the  Lumiere  Co. 


2J2 


PHOTOGRAPHY  IN  COLOURS 


Quantities  sufficient  for  whole 
plate  or  7*  X 5. 

Time  of  action. 

Remarks. 

B. — Reversing  bath — 

Potassium  Perman- 
ganate, 0-2  grm.  (3 
grains) 

Sulphuric  Acid  1 c.c. 
(17  m.) 

Water,  100  c.c.  (3| 
oz.) 

2-4  min.  ac- 
cording to 
appearance 
of  image  as 
observed 
from  time 
to  time  in 
daylight 

After  plate  has  been 
in  B for  J min.,  the 
dish  may  be  carried 
into  full  daylight  and 
the  transparency  ex- 
amined. Wash  for  1 
minute  in  several 
changes  of  water. 

C. — Redevelopment — 
Plate  is  returned  to 
AA  bath 

4 min. 

Must  be  carried  out  in 
full  daylight  or  nega- 
tive must  first  be  ex- 
posed six  inches  in 
front  of  1 ft.  of  Mag- 
nesium ribbon.  Wash 
2 mins. 

D. — Hardening  and 

clearing  bath — 
Powdered  Chrome 
Alum,  6 grms.  (90 
grns.),  or  alum  6 
grms.,  Citric  Acid 
in  powder  0-6  grm. 
(9  grns.) 

Tap  water,  4 ounces. 
Quantities  need 

not  be  measured. 

This  bath  is  optional, 
but  brightens  up  and 
toughens  the  film. 
Wash  2 minutes. 
Then  leave  to  dry. 
Fixing  in  Hypo  op- 
tional. (Wash  3 

minutes.) 

Or  instead  use  a 1/500  bath  of  Permanganate  of  Potassium 
(without  acid).  This  is  preferred  by  Lumiere  to  the  alum  and 
citric  bath.  In  hot  weather  Chrome  Alum  must  be  used. 


9.  Lumiere’s  Improved  Graduated  Developer  for 
Uncertain  Exposures  (Latest  Formula,  1910). 

This  developer  allows  of  greater  latitude  in  exposure 
than  the  last  mentioned.  The  other  baths  remain  the 


same. 


APPENDIX 


273 


Solution  1. — Place  in  glass  measure  (for  a quarter- 
plate  or  5x4)  40  c.c.  (1  fl.  oz.  2 drms.)  of  water. 
Add  Quinomet  (concentrated  developer)  2-5  c.c.  (42  m.). 
Call  this  A. 

Temperature  about  15°  C.  (60°  F.). 

Solution  2. — Also  in  a second  (small)  measure  put 
7*5  c.c.  (2  drms.  8 m.)  of  concentrated  developer.  Call 
this  B. 

Place  your  watch  close  to  green  (Yirida  paper)  safelight. 

Put  plate  in  dish  in  nearly  total  darkness,  and  pour 
developer  A over  it  the  moment  the  second  hand  reaches 
60  seconds.  Rock  the  dish  (screened  from  the  light)  for 
14  seconds,  then  bring  the  dish  containing  plate  close 
to  the  light  for  a moment  to  see  if  a trace  of  the  image 
has  begun  to  appear  (ignoring  sky).  The  moment  this 
occurs  note  number  of  seconds  which  have  elapsed. 
Then  add  immediately  contents  of  small  measure  (B) 
while  rocking  dish.  Put  cover  over  dish  and  consult 
the  following  table,  which  you  should  have  previously 
written  in  Indian  ink  on  the  outermost  Yirida  paper  of 
the  lamp. 


If  time  which  has  elapsed 

Development  should  be  continued 

since  first  appearance 

(from  the  commencement)  for 

of  image  is 

minutes. 

seconds. 

L 2"  to  14"  . . . 

.....  1 

15 

15"  to  17"  . . . 

1 

45 

18"  to  21"  . . . 

2 

15 

22"  to  27"  . . . 

3 

0 

28"  to  33"  . . . 

3 

30 

34"  to  39" 

4 

30 

If  image  fails  to  appear  after  40",  add  22  c.c.  of 
Quinomet  solution  (B). 

minutes,  seconds. 

. 3 0 

. 4 0 

T 


40"  to  47" 
48"  to  60" 


274  PHOTOGRAPHY  IN  COLOURS 


If  no  image  appears  at  the  end  of  a minute, 
the  plate  is  hopelessly  under-exposed.  Then  wash 
in  the  dark,  place  in  reversal  bath  and  redevelop  accord- 
ing to  instructions  given  in  Table  8.  If  after  reversal  the 
image  looks  heavy  and  dull,  i.e.  wanting  transparency, 
it  shows  under-exposure.  This  can  sometimes  be  partly 
remedied  by  placing  for  a moment  in  a fresh  hypo 
bath  and  looked  at  frequently,  until  the  colours  are 
more  transparent.  Then  wash  well,  redevelop,  and 
intensify. 

In  hot  weather,  or  if  temperature  of  the  solutions 
exceeds  68°  F.,  use  Chrome  Alum  bath  immediately 
after  reversal  to  prevent  frilling. 

Eeduction  or  intensification  is  preferably  carried  out 
immediately  after  the  redevelopment  (before  positive  has 
begun  to  dry).  After  intensification  it  must  be  immersed 
in  the  fixing  bath,  otherwise  the  latter  bath  may  be  dis- 
pensed with.  If  the  second  development  has  not  been 
very  thorough,  and  no  intensification  performed,  fixa- 
tion had  better  be  omitted,  as  there  is  risk  of  producing 
a flat,  dull  image. 

As  Professor  Namias’s  modification  of  Lumiere’s 
formula  is  very  highly  spoken  of  by  many  amateurs,  I 
give  it  here. 

9a.  Professor  Namias’s  Method  of  Developing 
Autochromes. — Make  the  following  stock  solution  : — 

Soda  sulphite  (crystals)  ....  100  grms.  (or  2 oz.) 

Ammonia  22  per  cent.  (Beaume)  . 32  c.c.  (4£  drams) 

Pot.  bromide 6 grms.  (54  grs.) 

Metol 4 grms.  (35  grs.) 

Hydroquinone 12  grms.  (105  grs.) 

Water 1000  c.c.  (20  oz.) 


APPENDIX 


275 


After  reversal,  clear  in  the  following  solution  : — 
Potassium  ferricyanide  3 per  cent,  solution  . 50  c.c.  (3  oz.) 


Ammonia 4 ,,  (2  drms.) 

Hypo  10  per  cent,  solution 50  ,,  (3  oz.) 


Dilute  with  twice  its  bulk  (200  c.c.)  of  water. 
Intensify  by  the  mercury  method  : — 


Bichloride  of  mercury 05  grm. 

Common  salt 1 ,, 

Hydrochloric  acid a few  drops 

Water  . . . 100  c.c. 


Then  rinse  well  and  re-develop  in  the  original 
developer.  If  less  effect  be  desired,  use  a 5 per  cent, 
solution  of  sodium  sulphite.  In  commencing  develop- 
ment Professor  Namias  uses  two  baths  (1st)  5 c.c.  of 
the  above-mentioned  stock  solution  to  50  c.c.  of  water, 
and  (2nd)  20  c.c.  of  stock  solution  to  80  c.c.  of  water. 
He  immerses  in  first  solution  to  find  the  time  which 
has  expired  before  the  image  begins  to  appear  (neglect- 
ing the  sky),  and  then  he  immediately  puts  the  plate 
in  the  second  solution  and  develops  by  Lumiere’s  table 
( vide  previous  section,  p.  273,  Table  9). 

10.  Other  Developers. 

Instead  of  the  Pyro  or  Quinomet  solutions,  Rodinal 
may  be  used,  diluted  1 to  5 of  water  for  under-exposed 
plates,  or  1 to  6 for  normal  exposures,  and  1 to  15  or 
1 to  20  for  over-exposed  plates.  Develop  for  about  2 
minutes,  or,  with  diluted  developer,  for  4 to  8 minutes. 
Rodinal  is  a very  powerful  developer,  and  is  eminently 
suited  for  travelling,  as  it  occupies  so  little  space. 
Ordinary  tap  water  may  be  used.  Of  course,  any 


276  PHOTOGRAPHY  IN  COLOURS 


one  of  the  developers  mentioned  in  the  Table  of 
Factor  Numbers  (No.  6)  may  be  used,  and  the  time  of 
development  regulated  by  the  factor  number.  Among 
those  specially  recommended  are  Metol-bydroquinone 
(Meto-quinol),  Glycin,  Eytol,  and  Rodinal  (1-20  or  1*30 
of  water).  This  last  developer  is  highly  recommended 
and  very  useful  for  travelling,  as  it  takes  up  no  room 
and  requires  no  acceleration. 

The  Author  has  discarded  Lumiere’s  graduated  de- 
velopment method  and  prefers  Meto-quinol,  using  12 
as  the  factor  number.  He  covers  the  dish  with  a card 
the  moment  the  image  begins  to  appear,  and  does  not 
uncover  it  until  at  least  two-thirds  of  the  time  has 
elapsed. 

11.  Intensification  Foemul^e. 


Lumiere  recommends  the  following  : — 


(1)  Pyrogallic  Acid  . 
Citric  Acid  . 
Distilled  Water 

(2)  Nitrate  of  Silver 
Distilled  Water 


3 grms.  \ 

3 grms.  ? Label  P. 
100  c.c.  * 

iroSlLabelG- 


For  use,  take  5 c.c.  of  G and  add  5 c.c.  of  F ; pour  into 
50  c.c.  of  distilled  water.  Pour  this  over  positive,  and 
rock  until  colours  are  sufficiently  bright,  or  until  the 
solution  becomes  turbid.  Be  sure  you  use  the  solution 
the  moment  you  have  added  the  silver  to  solution  F, 
for  precipitation  of  the  silver  commences  in  about  half 
a minute,  and  the  solution  becomes  black  and  useless. 

Another  excellent  method  is  : — 


(1)  Perchloride  of  Mercury  ....  \ grm. 

Common  Salt 1 grm. 

Water  100  c.c. 


APPENDIX 


2 77 


(2)  Sulphite  of  Soda  (Oryst.)  ...  5 grins. 

Water 100  c.c. 

These  baths  may  be  used  repeatedly. 

Leave  plate  in  (1)  until  the  image  is  completely 
whitened.  Wash  for  a minute  and  place  in  sulphite 
solution.  The  process  may  be  repeated  if  more  inten- 
sification be  required,  and  one  may  either  redevelop 
with  amidol  or  metol-hydroquinone  developer,  or  1 % 
of  strong  ammonia  solution  may  be  used  instead  of  the 
sulphite  bath.  This  gives  a very  black  image.  Then 
fix  in  hypo. 

Either  of  the  above  methods  can  be  used  to  intensify 
the  image  or  any  of  the  plates  mentioned. 

As  soon  as  the  image  has  been  intensified  it  must  be 
fixed  in  hypo,  since  during  intensification  in  either 
method  a silver  compound  is  formed  which  will  be 
acted  upon  by  the  light. 

12.  Reduction  Foemul2e. 

(1)  Either  use  the  acid  permanganate  or  the  acid 
bichromate  reversal  bath  diluted  1 to  10,  or  use — 

(2)  Farmer's  solution , viz. : — 

Hyposulphite  of  Soda 15  grms. 

Ferricyanide  of  Potassium  ...  1 grm. 

Water 100  c.c. 

This  gives  an  even  reduction  all  over  the  plate.  Or — 

(3)  Persulphate  reducer , viz. : — 

Persulphate  of  Ammonia  ...  2 grms. 

Water  100  c.c. 

This  latter  reducer  acts  first  on  the  denser  parts  and 
leaves  the  half-tones  unaffected,  and  is  for  this  reason 


278  PHOTOGRAPHY  IN  COLOURS 

to  be  preferred  to  the  others  when  selective  action  is 
desired.  It  softens  a hard  negative.  Rock  the  dish 
and  examine  the  plate  every  half-minnte.  Wash 
immediately  the  reduction  is  sufficient. 

N.B. — Use  the  moment  the  bath  is  made  up,  as  it 
does  not  keep. 

Stop  the  action  as  soon  as  the  image  viewed  by 
transmitted  daylight  is  clear  enough.  Then,  if  neces- 
sary, re-intensify  a little  to  bring  up  the  colours. 

After  intensification,  clean  in  neutral  permanganate 
bath  (same  as  reversal  bath  for  Lumiere  plate,  only  no 
acid  is  added).  Then  fix  in  hypo. 

13.  Insteuctions  foe  Developing  Omnicoloee 
Plates.  (Makees’  Foemul^.) 

Quantities  suitable  for  7|  x 5 to  whole  plate  size. 


Solutions. 

Duration. 

Eemarks. 

A. — ls£  Developer , 13°  C. 
to  18°  C.— 

Water  distilled,  100 

2 to  5 minutes 

Wash  for  20  seconds  in 

c.c.  (3|  oz.) 

Metol,  0-4  gm.  (6  grs.) 

Hydroquinone,  0*2 
gm.  (3  grs.) 

Sodium  Sulphite 

(anhyd.)  5 gms.  (77 
grs.) 

Carbonate  of  Potas- 
sium (dried)  3 gms. 
(46  grs.) 

Bromide  of  Potas- 
sium, 0*1  gm.  (1| 
grs.) 

Sodium  Hyposul- 

phite (1  % sol.),  1-5 
c.c.  (25  m.) 

the  dark. 

APPENDIX 


279 


Solutions. 

Duration. 

Remarks. 

B. — Reversal  Bath — 
Distilled  Water,  100 
c.c.  (3|  oz.) 
Bichromate  of  Potash 
or  Soda,  0-8  gm. 
(12i  grs.) 

Sulphuric  Acid,  1-2 
c.c.  (20  m.) 

2 minutes 

Wash  for  2 minutes  in 
the  light. 

C.  — Redeveloper  — Use 

1st  developer  (A) 
or  make  up  fresh 
developer 

D.  — Fixing  Bath — 
Water  (tap)  100  c.c. 
Hyposulphite  of  soda 

(crystal)  20  gms. 
Bisulphite  of  soda  (or 
metabisulphite)  3 
gms.  (46  grs.) 

Not  to  exceed 
2 minutes 

Leave  plate  in  solution 
in  bright  daylight 
until  it  is  quite  black. 
Usually  4 minutes 
will  suffice. 

This  bath  is  optional, 
but  many  workers 
consider  the  colours 
are  more  permanent 
and  the  positive  is 
less  liable  to  change 
if  it  is  used. 

Note. — Any  one  of  Lumiere’s  formulse  (Tables  7,  8 or  9)  may 
be  used  for  developing  Omnicolore  or  Dufay  plates,  and  the  Omni- 
colore formulse  may  be  used  for  Autochrome  plates.  After  the 
plate  has  been  in  the  reversal  bath  for  half  a minute,  it  should 
be  repeatedly  examined  in  artificial  light  so  as  to  stop  the 
moment  all  details  are  visible,  otherwise  the  high  lights  may  be 
eaten  away. 

The  plate  must  be  thoroughly  washed  for  2 minutes 
after  reversal  to  get  all  the  bichromate  out.  It  is 
advisable  to  put  the  plate  for  one  minute  in  a 1 % 
solution  of  sodium  bisulphite  after  washing.  Then 
place  in  the  old  developing-bath  and  redevelop  in  bright 
daylight.  If  the  plate  is  veiled  or  grey,  place  in  a little 
of  the  old  reversing  solution,  diluted  1 to  100  with  water 


280  photography  in  colours 


for  20  seconds  or  so.  Then  wash  and  leave  to  dry.  If 
the  colours  are  not  bright  enough,  intensify  and,  after 
washing,  fix  in  hypo  and  again  wash  thoroughly.  If  the 
image  is  not  intensified,  the  hypo  bath  may  be  omitted. 

14.  Instructions  for  Developing  the  Dufay 
Plate. 

The  instructions  given  for  the  Omnicolore  plate  will 
answer  perfectly  for  the  Dufay.  Or  Lumiere’s  im- 
proved Quinomet  developer  will  answer  equally,  but 
it  is  recommended  to  use  the  acid  bichromate  reverser 
instead  of  the  permanganate,  as  it  has  greater  pene- 
trating power — although  the  latter  may  be  used  quite 
successfully. 

The  instructions  given  with  the  Dufay  plate  are  as 
follows : — 


A.  Developer 

Metol 

Sulphite  of  Soda  crystallised 

• 75  „ 

Hydroquinone 

2 „ 

Bromide  of  Potassium 

2 „ 

Ammonia,  -880  

Water 

. 1000  „ 

For  use  dilute  with  equal  parts  of  water. 

For  a correctly  exposed  plate,  develop  for  4 to  5 
minutes. 

As  soon  as  all  the  details  have  shown  up,  wash  for 
20  seconds  and  place  in — 


B.  Reversal  Bath 

Bichromate  of  Soda  or  Potash  . . 5 grms. 

Sulphuric  Acid  10  c.c. 

Water 1000  „ 


APPENDIX 


28l 


The  moment  the  plate  is  covered  by  this  solution,  take 
the  dish  and  negative  into  daylight,  and  hold  up  the 
plate  for  a moment  from  time  to  time  and  examine  by 
transmitted  light,  until  the  image  appears  quite  distinct 
in  its  natural  colours.  Then  wash  under  the  tap  imme- 
diately. If  this  is  delayed  too  long  the  highlights  will 
be  eaten  away.  The  usual  time  for  this  bath  is  2 
minutes. 

Then  wash  for  a minute  or  two  and  place  in  a 10  % 
sulphite  of  soda  bath.  If  the  air  or  water  is  too 
warm  ( i.e . over  68°),  place  the  plate  in  a solution  of 
chrome  alum  for  a minute  or  two.  Then  redevelop  in 
bright  daylight  in  the  first  bath  (A)  until  the  plate  is 
quite  black. 

For  intensification,  soak  first  in  a saturated  solution 
of  perchloride  of  mercury  for  5 minutes,  or  until  quite 
bleached  through. 

Perchloride  of  Mercury  (Sublimate)  40  grms. 

Alcohol  (Meth.  Spirit) .....  200  „ 

Water 800  c.c. 

Then  into  a 10  % solution  of  sulphite  of  soda  in 
water. 

These  two  baths  can  be  repeated  if  desired,  and  may 
be  used  over  and  over  again. 

For  under-exposure  the  density  may  be  reduced  by 
Farmer’s  solution  (see  Table  12),  or  the  Persulphite 
reducer,  or  the  reversal  bath  diluted  1 : 10  or  1 : 15 
may  be  used,  if  the  Hypo  bath  be  objected  to. 


282 


PHOTOGRAPHY  IN  COLOURS 


15.  Instructions  for  Developing  Paget  Plates. 

Development. 

After  exposure  the  Panchromatic  Plate  should  be 
taken  from  the  dark  slide  and  developed  in  the  ordinary 
way.  The  Taking  Screen  will,  of  course,  be  kept  for 
future  exposures. 

Most  developers  may  be  used,  provided  the  resulting 
negative  be  clean  and  soft.  The  best  results  are 
obtained  with  Rodinal,  1 in  30,  and  development 
should  be  complete  in  two  minutes. 

Unless  a Green  Safelight  is  used  development  must 
take  place  in  total  darkness.  On  no  account  should  a 
Red  Light  or  one  of  any  colour  other  than  the  Safe 
Green  be  used.  Development  in  total  darkness  pre- 
sents no  difficulty,  as  if  the  exposure  given  is  about 
right,  the  time  of  development  with  Rodinal  as  given 
above  will  be  correct. 

Rinse  the  plate  and  fix  in  the  following  bath : — 


Hypo 6 ozs. 

Potass.  Metabisulphite \ oz. 

Water 20  ozs. 


Wash  again  for  about  15  minutes,  and  put  to  dry. 


Making  the  Transparency . 

From  the  negative  made  in  accordance  with  the 
foregoing  instructions  a contact  transparency  is  made, 
and  to  obtain  the  best  results  the  following  conditions 
must  be  observed. 

The  Transparency  should  be  of  black  tone,  perfectly 


APPENDIX 


283 

clear,  and  free  from  fog,  brilliant  and  full  of  detail. 
These  conditions  can  be  secured  by  using  the  special 
transparency  plates  issued  in  connection  with  this 
process,  and  adhering  to  the  instructions  therewith. 

Registration. 

When  the  Transparency  is  dry  it  is  ready  to  be 
registered  with  the  Viewing  Screen.  The  method  of 
doing  this  is  to  place  the  Transparency  upon  the 
Viewing  Screen,  film  to  film,  and  holding  them  up 
together  in  this  position  so  as  to  look  through.  Keep- 
ing one  of  the  plates  stationary,  move  the  other  about 
slowly,  maintaining  contact  all  the  time,  and  altering 
the  position  by  minute  steps,  until  all  signs  of  any 
pattern  have  disappeared,  when  a slight  movement 
either  to  the  right  or  left  will  show  the  picture  in  its 
correct  colours.  Clip  the  plates  together  with  strong 
letter  clips  and  bind  in  the  same  way  as  with  ordinary 
lantern  plates. 


16.  Elimination  of  Green  Spots. 

Mr.  J.  McIntosh  recommends  that  the  green  spots 
should  be  cut  out  with  a sharp  knife,  and  then  a 
lantern  plate  exposed  in  contact  with  it.  The  ex- 
posure must  be  very  brief.  On  development,  a grey 
spot  will  be  seen  corresponding  in  position  and  size  to 
the  hole.  After  fixing  and  drying  it  the  spot  can  be 
retouched  and  worked  up  with  Aniline  colours  to 
harmonise  with  the  picture.  The  negative  must  then 
be  bound  up  with  the  positive  film  to  film. 


284  PHOTOGRAPHY  IN  COLOURS 


17.  Recent  Devices  for  Protecting  the  Colour 

Slide  from  the  Heat  Rays  of  the  Lantern. 

M.  Massiot  protects  the  slide  by  separating  the  two 
halves  of  the  condenser  by  means  of  a freely  ventilated 
wooden  box,  which  is  placed  outside  the  lantern.  It 
measures  about  8"  or  10"  long.  In  order  to  further 
diminish  the  heat,  the  to  and  fro  carrier  is  provided 
with  two  converging  lenses  of  a focus  selected  to  suit 
the  objective.  These  lenses  being  thrown  outside  the 
path  of  the  rays  with  each  change  of  slide,  have  time 
to  cool,  and  thus  become  only  moderately  heated.  It 
must  be  explained  that  each  lens  attached  to  the 
carrier  really  forms  part  of  the  condensing  system, 
and  its  addition  is  necessary  to  completely  fill  the  slide 
with  light.  Zeiss  in  his  Epidiascope  projecting  lantern 
has  quite  overcome  the  difficulty  by  employing  a 
mirror  which  reflects  the  light  through  the  trans- 
parency at  such  a distance  from  the  source  as  to 
render  it  perfectly  safe  from  injury.  It  is  now  sold  in 
Paris  under  the  name  of  the  “ Erigida  ” projecting 
lantern.  It  is,  moreover,  a very  much  cheaper  form  of 
lantern  than  the  Zeiss  model.  A somewhat  similar 
device  is  fitted  to  the  projecting  lantern  sold  by  the 
firm  of  Bouch  and  Lomb,  which  may  be  obtained  from 
Staley  and  Co.,  24,  Thavies  Inn,  London. 

18.  Sensitising  and  Resensitising  Colour 
Plates. 

Dr.  Konig,  in  a recent  number  of  the  “ Photogr. 
Rundschau,"  has  strongly  recommended  the  following 
sensitising  bath  : — 


APPENDIX 


285 


Alcohol 

Pina  chrome -violet  (1  : 1000) 

Orthochrome,  Pinaverdol,  or  Pinachrome  (1  : 1000) 

Water  (distilled) 

Bathe  for  three  minutes  ; do  not  wash. 

The  doctor  points  out  the  immense  superiority  of 
these  dyes  over  pinachrome  behind  a red  filter,  and 
over  a mixture  of  pinacyanol  and  orthochrome  when 
exposing  behind  a green  filter. 

19.  Colour- screen  Filters  and  Monochromatic 
Light. 

' The  Mercury  vapour  spectrum  yields  the  following 
lines : — 

Yellow  . . . Wave-length  579  /x/x  and  576  /x/x 

Green  ...  „ ,,  54:6 /x/x 

Blue  ....  ,,  ,,  436  jxjx 

Violet.  ...  ,,  ,,  407  /x/x  and  405  /x/x 

Bed  is  entirely  absent. 

The  three  following  filters  will  be  found  useful  with 
this  lamp : — 

To  transmit  yellow  light  only  of  A = 579  ^ and  576  /x/x 

Potas.  bichrom 15  grms. 

Copper  sulphate 3*5  grms. 

Sulphuric  acid 1 c.c. 

Distilled  water 300  c.c. 


100  c.c. 
3 c.c. 
3 c.c. 
200  c.c. 


To  transmit  green  light,  of  A = 546  /x/x 


Picric  acid 0-4  grm. 

Copper  sulphate 3-5  grm. 

Didymium  nitrate  ....  15  grms. 

Water 300  c.c. 


286  PHOTOGRAPHY  IN  COLOURS 


To  transmit  blue  light  only  of  A = 407  fifx  and  405  ^ 

Copper  sulphate 1 grm. 

Distilled  water 225  c.c. 

Ammonia  (0*880)  ....  75  c.e. 

20.  A.  B.  Hitchins’  Developer. 

Owing  to  his  vast  experience  in  colour  portraiture 
this  formula  can  be  recommended  with  every  con- 
fidence : — 

Metol 6*5  grms. 

Sod.  sulphite 40  „ 

Hydroquinone  ....  2*1  ,, 

Pot.  bromide 2*5  „ 

Sod.  hyposulphite  ...  0*1  gram. 

Ammonia  (0*880)  ...  20  c.c. 

Water 1000  ,, 

Carry  on  development  until  the  high  lights  and 
flesh  tones  just  begin  to  show  reversal  and  trans- 
parency when  viewed  against  the  green  Yirida  safe- 
light,  i.e.  in  about  3 to  4 minutes.  Rinse  in  water 


and  then  place  in  reverser.  This  is  best  made  up  as 
follows : — 

Potas.  bichrom 4 grms. 

Sulphuric  acid 15  c.c. 

Water 1000  ,, 

Then  redevelop  with  — 

Sod.  sulphite  (anhydrous)  ...  21  grms. 

Diamidophenol 6 grms. 

Pot.  bromide  (10%  solution)  . . 100  minims. 

Water 1000  c.c. 


Continue  development  for  4 minutes.  Temperature 
of  water  65  F. 

N.B. — If  a very  clear  transparent  positive  be  desired, 


APPENDIX 


287 

or  for  lantern  exhibition,  add  to  first  developer 
4*7  grms.  of  Ferrocyanide  of  Potassium  ( not  Ferri- 
cyanide ),  and  omit  the  Hypo. 


21.  Metric  Equivalent  Tables. 


Solid  measures  (Metric). 

Solid  measures  (British). 

1 Milligram 

= e h srain 

1 grain 

= 65  milligrams 

1 Centigram =^7?= 0-154  grain 

2 

3 

11 

11 

= 13  centigrams 
= 19-5  „ 

1 Decigram 

= 1-543  grains 

4 

11 

= 26 

11 

0-1  Gramme 

= 1-5 

5 

11 

= 32-4 

11 

0-2 

=3 

6 

11 

= 39 

11 

0-3 

= 4± 

7 

11 

= 45 

11 

0-4 

=6 

8 

11 

= 52 

11 

0*5 

= 7i 

9 

11 

= 58 

11 

0-6 

= 9“ 

10 

„ 

= 65 

11 

0-7 

= 11 

11 

11 

= 72 

11 

0-8 

= 12J 

12 

11 

= 78 

11 

0*9 

= 14 

13 

11 

=84-5 

11 

1 

= 15-43  „ 

14 

11 

= 91 

11 

2 

= 31 

15 

„ 

= 97-4 

11 

3 

= 46 

16 

11 

= 1-04  grammes 

4 

= 62 

17 

11 

= 1-1 

11 

5 

= 77 

18 

11 

= 1-17 

11 

6 

= 92-5 

19 

11 

= 1-23 

11 

7 

= 108 

20 

11 

= 1-3 

11 

8 

= 123 

30 

}) 

= 1-95 

11 

9 

= 139 

40 

11 

= 2-6 

11 

10 

) j 

= 154 

50 

11 

= 3-24 

11 

14 

11 

= 216  = | oz.  avoir. 

60 

11 

= 3-9 

11 

20 

11 

= 308  grains 

I' 

dz.  avoir. 

= 7 

11 

28 

11 

= 437  grains  = 1 oz.  av. 

1 

2 

11  11 

= 14-17 

11 

1 

11  11 

= 28-35 

1 1 

1 lb.  = 16  oz.  = 454 

11 

Note. — Gramme  is  generally  written  “ gr.,”  but  English 
writers  usually  indicate  it  by  gm.  or  grm.,  to  distinguish  it 
from  gr.  (grain),  but  the  occurrence  of  c.c.  in  the  one  case,  or 
ounces  and  minims  in  the  other  case,  will  enable  the  reader  to 
know  at  once  which  measure  “ gr.  ” stands  for.  On  the  Conti- 
nent all  liquids  as  well  as  solids  are  sold  by  weight  in  grammes. 


288  PHOTOGRAPHY  IN  COLOURS 


Fluid  measures  (Metric). 


1 C.C.: 

2 „ = 

3 „ : 
3-5  „ : 

4 „ 

5 „ 


7 

8 
9 

10 

11 

12 

13 

14 

15 
20 
25 
28 
30 
40 
50 
75 

100 

1000 


;17  minims  (m.) 

34  „ 

51  „ 

:60=3i  (1  drachm) 
:68  minims 
:85  „ 

- 1 dr.  41  m. 

;2  „ 

:2  „ 15  „ 

;2| ,, 

:2  „ 49  „ 

" 3 „ 6 „ 

3 „ 23  „ 

■ 3 „ 40  „ 

A „ 

:4  ,,  14  „ 

= 5 „ 8 ,, 

» 

:§i(lfl.oz.)  = 480m. 
= 8|  dr. 

:1  fl.  oz.  2 drms.) 
:§i3vi  (lfl.oz.  6 dr.) 

:fiiis  (3|  fl.  oz.) 

: 1 litre  = 35-2  fl.  oz. 


Fluid  measures  (British). 


drachm  (3i) 


1 minim  (m.)  = c.c.  = 0*06  c.c. 

5 ,,  = 0*29 ,, 

0- 59  „ 

1- 18  „ 

1- 77  „ 

2- 36  „ 

2*95  „ 

3- 5  „ 

7 „ 

r m =9  „ 

„ =10*65  c.c. 

„ (i  fl.oz)  = 14 
„ =17*5 

„ =21*3 

„ =24*7 

fl.  oz.  (3i)  = 28 

„ (|ii)  = 57 

„ (iiii)  = 85 
3^  „ (3iiiss)  = 100 
4 „ (Biv)  =113 

1 pint  (Oi)  =568 
35*2  fl.  oz.  =1000  c.c.  = 1 litre 
1 quart  =1*136  litres 
1 gallon  =4*546  litres 


Conversion  of  grammes  per  litre  into  grains  per 
ounce : multiply  the  grammes  by  0*44,  product  is 
grains  per  ounce.  For  c.c.  per  litre  into  minims  per 
ounce,  multiply  by  0*48.  Conversion  of  grains  per  ounce 
into  grammes  per  litre : multiply  grains  by  2*3,  product 
is  grammes  per  litre.  Thus  40  grs.  in  16  ozs.  = 2J  grs. 
per  oz.,  and  2*5  x 2*3  = 5*75  grammes  per  litre.  For 
minims  per  ounce  into  c.c.  per  litre,  multiply  the  num- 
ber of  minims  by  2*3.  Thus  20  mm.  in  4 ozs.  = 5 m. 
per  ounce,  and  5 x 2*3  = 11*5  grammes  per  litre. 


APPENDIX 


289 


| mile. 


Measures  of  Length  (Metric). 

1 Kilometre  = 1000  M.  = 1094  yards 

1 Metre  (M.)  = 10  decimetres  = 100  cm.  = 39-37  in. 

1 Decimetre  (dm.)  = 10  cm.  = 3*937  in. 

1 Centimetre  (cm.)  = 10  mm.  = 0-3937  in. 

1 Millimetre  (mm.)  = 1000  microns  = Jg-  in.  = 0*03937  in. 

1 Micron  (/ j .)  = 1000  micromillimetres  = 2 5troo  in- 

1 Micromillimetre  (hi/j.)  = lOAng-) _ j 

strom  units  (often  written  A.U .)  J — 1 0 0 0 0 0 0 m 7T1 


25000000 


Measures  of  Length  (British). 


1 mile  = 1609  M. 

1 furlong  = 201  M. 

1 yard  = 91*41  cm. 


1 foot  =.  30*47  cm. 
1 inch  = 25*4  mm. 
1 line  = 2 mm. 


Inches  to  millimetres. 


Inches 

mm. 

cm. 

ft 

= 

1*58  = 

0*16 

8 

= 

3*17  = 

0*32 

1 

4 

— 

6*35  = 

0*63 

3 

8 

=r 

9*5  = 

0*95 

i 

— 

12*7  = 

1*27 

1 

— 

15*9  = 

1*59 

3 

■4 

— 

19 

1*9 

1 

8 

— 

22*2  = 

2*2 

1 

= - 

25*4  = 

2*54 

2 

= 

50*8  = 

5*08 

3 

— 

76*2  = 

7*6 

4 

=: 

101*6  = 

10*1 

5 

= 

127 

12*7 

6 

= 

152  = 

15*2 

7 

— 

177 

17*7 

8 

— 

203 

20*3 

9 

= 

229 

22*9 

10 

— 

254 

25*4 

11 

— 

280 

28 

12 

— 

304 

30*4 

13 

— 

330  = 

33 

14 

— 

355  = 

35*5 

15 

= 

381  = 

38*1 

16 

= 

406  = 

40*6 

17 

= 

431 

43*1 

18 

— 

458  = 

45*8 

19 

rrz 

483  = 

48*3 

20 

— 

508 

50*8 

The  above  values  are  correct  to 
^ mm. 


Centimetres  to  inches . 


cm.  inches. 


1 

— 

3 

g 

2 

= 

a 

3 

= 

1 i'i; 

4 

= 

tH  rH 

5 

= 

6 

= 

21 

7 

2f 

8 

— 

3* 

9 

= 

8ft 

10 

— 

3U 

11 

4t56 

12 

= 

4ft 

13 

— 

14 

= 

15 

k15 

16 

= 

6ft 

17 

= 

611 

6 

18 

= 

7ft 

n 

19 

— 

20 

=: 

'll 

21 

— 

8i 

22 

= 

8| 

23 

■J= 

9 

24 

= 

9§ 

25 

= 

9^ 

26 

— 

101 

27 

= 

10f 

28 

= 

11 

The  above  values  are  correct  to 

ft  m. 

U 


290  PHOTOGRAPHY  IN  COLOURS 


22.  English  and  Foreign  Sizes  of  Plates. 


Continental  Sizes  of  Plates. 


Centimetres. 

Inches. 

Centimetres. 

Inches. 

4*5  X 6*0 

If  X 2f 

13  X 21 

5*12  X 8-25 

9 X 12 

3-54  X 4*72 

18  X 24 

5-12  X 8-25 

12  X 16 

4*72  X 6*30 

24  X 30 

9*44  X 11-80 

13  X 18 

5-12  X 7*08 

30  X 40 

11-80  X 15*75 

English  Sizes  of  Plates. 

Inches. 

Centimetres. 

Inches. 

Centimetres. 

X 2J 

8*9  X 6-4 

7X5 

17*8  X 12-7 

3 1 X 3i 

8-25  X 8*25 

8§  X 6± 

21*5  X 16-5 

4£X  8*  | 

10-8  X 8-25 

10  X 8 

25*4  X 20-3 

5X4 

12-6  X 10-1 

12  X 10 

30-4  X 25*4 

6|  X 4f 

16-5  X 12-0 

15  X 12 

38-1  X 30-4 

23.  Comparative  Plate  Speeds. 


H.  and  D. 

Watkins. 

Wynne. 

1 

1J 

8 

2 

3 

11 

3 

4,5 

14 

4 

6 

16 

5 

n 

18 

8 

12 

22 

10 

15 

24 

N.B. — Hurter  and  Driffield’s  2,  Watkin’s  8,  and  Wynne’s  Meter 
11  correspond  to  Wellcome’s  Plate  Speed  No.  12,  which  is  the 
correct  number  for  Autochrome  and  Dufay  plates  out  of  doors. 

The  makers  give  for  Autochrome  Plates  Watkin’s  3 and 
Wynne  11  and  for  Paget  Plates  (separate),  Watkin’s  7£  and 
Wynne  18.  In  both  cases  with  filter  in  position. 


APPENDIX 


291 


24.  Wave  Lengths  of  Visible  Spectrum. 


768/^Visible  limit  of  spectrum 


Wave  length  of  dark  red 

A line 

= 759 

Oxygen  line 

11 

11 

deep  red 

a 

11 

= 738 

Water  vapour 

11 

11 

red 

B 

11 

= 687 

Oxygen  „ 

11 

11 

light  red 

— 

11 

= 670 

Lithium  „ 

11 

11 

orange  red 

0 

11 

= 656 

Hydrogen  vapour 

11 

11 

yellow 

D 

11 

= 589 

Sodium  „ 

11 

„ 

green 

E 

11 

= 527 

Iron  ,, 

11 

11 

bluish- green 

* b 

11 

= 518 

Magnesium  ,, 

11 

11 

greenish-blue 

F 

11 

= 486 

Hydrogen  „ 

11 

11 

blue 

— 

11 

= 460 

Lithium  „ 

11 

11 

blue-violet 

G 

11 

= 434 

Iron  „ 

11 

11 

violet 

H 

11 

= 397 

Visible  limit  of  spectrum 

Calcium  vapour 

K 

11 

= 393 

Ultra-violet  begins 

Roughly  speaking,  blue  extends  from  400  to  500,  green  and 


yellow  from  500  to  600,  red  from  600  to  700  n/x. 


292 


PHOTOGRAPHY  IN  COLOURS 


25.  Thermometric  Scales 


Centigrade, 
(Celsius)  C. 

Fahrenheit 

F. 

Reaumur 

R. 

Centigrade 

C. 

Fahrenheit 

F. 

Reaumur 

R. 

0° 

32° 

0° 

26 

78,8 

20,8 

1 

33,8 

0,8 

27 

80,6 

21,6 

2 

35,6 

1,6 

28 

82,4 

22,4 

3 

37,4 

2,4 

29 

84,2 

23,2 

4 

39,2 

3,2 

30 

86,0 

24,0 

5 

41,0 

4,0 

31 

87,8 

24,8 

6 

42,8 

4,8 

32 

89,6 

25,6 

7 

44,6 

5,6 

33 

91,4 

26,4 

8 

46,4 

6,4 

34 

93,2 

27,2 

9 

48,2 

7,2 

35 

95,0 

28,0 

10 

50,0 

8,0 

36 

96,8 

28,8 

11 

51,8 

8,8 

37 

98,6 

29,6 

12 

53,6 

9,6 

38 

100,4 

30,4 

13 

55,4 

10,4 

39 

102,2 

31,2 

14 

57,2 

11,2 

40* 

104 

32 

15 

59,0 

12,0 

45 

113 

36 

16 

60,8 

12,8 

50 

122 

40 

17 

62,6 

13,6 

55 

131 

44 

18 

64,4 

14,4 

60 

140 

48 

19 

66,2 

15,2 

65 

149 

52 

20 

68,0 

16,0 

70 

158 

56 

21 

69,8 

16,8 

75 

167 

60 

22 

71,6 

17,6 

80 

176 

64 

23 

73,4 

18,4 

85 

185 

68 

24 

75,2 

19,2 

90 

194 

72 

25 

77,0 

20,0 

95 

203 

76 

100° 

212° 

80° 

Rule. — To  convert — 

0°  into  F°.  Multiply  C°  by  9,  divide  by  5,  and  add  32. 

R°  into  F°.  Multiply  R°  by  9,  divide  by  4,  and  add  32. 

C°  into  R°.  Multiply  C°  by  4,  and  divide  by  5. 

R°  into  C°.  Multiply  R°  by  5,  and  divide  by  4. 

F°  into  C°.  Subtract  32  from  F°,  multiply  remainder  by  5,  and 
divide  by  9. 

F°  into  R°.  Subtract  32  from  F°,  multiply  remainder  by  4,  and 
divide  by  9. 

Note. — Fahrenheit’s  scale  is  only  used  in  English-speaking 
countries.  Reaumur’s  scale  is  used  by  the  general  public  in  most 
countries  on  the  Continent.  The  Centigrade  scale  is  now  used  in 
all  countries  by  physicists  and  chemists,  and  this  scale  is  therefore 
implied  in  scientific  works  unless  otherwise  specially  mentioned. 


APPENDIX 


293 


26.  List  of  all  the  Firms  mentioned  in  this 
Work,  together  with  their  Postal  Addresses  and 
Telephone  Numbers.  (Lens  and  Camera  Makers 
are  omitted.) 


Name  of  Firm. 
Actien  Ges.  fur 
Analin  fabrika- 
tion. 

Autotype  Co. . 
Baker,  Charles  . 
Bayer  & Co.  . 
Berger  v.  Wirth  . 
Bohringer 
Burroughs  Well- 
come & Co. 
Butler,  H.  T.  . 

Dufay 


Fuerst  Bros.  . 

T.  K.  Grant  (suc- 
cessor to  Lu- 
miere Co. 

Griibler,  Chemi- 
ker. 

Ives  Inventions, 
Ltd. 

Johnson  & Sons  . 


Postal  Address. 


Telegraphic  Address  and 
Telephone  No. 


Berlin. 


74,  New  Oxford  St.,  W.C.  Central,  873 
244,  High  Holborn. 

Elberfeld,  Germany. 

Benthstrasse,  Berlin. 

Mannheim,  Germany. 

Snow-hill  Buildings,  Central,  13300. 

Holborn  Viaduct,  E.C. 

26,  Craven  Park,  Willes- 
den,  London,  N.W. 

22, RueChateaudun, Paris. 

London  Agents,  Auto- 
type Co.  ( q.v .) 

17,  Philpot  Lane,  E.C. 


89,  Great 
London, 


Bussell 

W.C. 


St., 


Fuerst,  London. 
LondonWall,4350. 
Diamido,  London. 
Gerrard,  3419. 


63,  Baierische  Strasse, 

Leipzig. 

939,  Eighth  Avenue,  New 
York. 

Mfg.  Chemists,  Ltd.,  23,  London  Wall,  677. 
Cross  St.,  E.C. 

Jougla,  J.  & Cie.  45,  Bue  de  Bivoli,  Paris  T.  N.,  105-75. 
Koenig,  Dr.  E.  See  Fuerst  Bros. 

Lumiere, A. &Sons  Monplaisir,  Lyon,  France  Lumiere,  Lyon. 


T.  N.,  11-19. 

Lumiere. A. & Sons  89, Great  Bussell  St., W.C.  Diamido,  London. 

Gerrard,  3419. 


294  PHOTOGRAPHY  IN  COLOURS 


Name  of  Firm. 

Dr.  G.  Mundie  . 

Natural  Colour 
Kinematograph 
Co.,  Ltd. 

Paget  Prize  Plate 
Co.,  Ltd. 

Raydex  Co.,  Ltd. 

Rotary  Photo  Co., 
Ltd. 

Royal  Photogra- 
phic Society 

Sanger-Shepherd 
& Co. 

Smith,  Dr.  J.  H. 

& Co. 


Postal  Address. 

Mik.  Chem.  Institut, 
Gottingen,  Hanover. 
Wardour  St.,  W. 


Watford,  England,  and 
244,  High  Holborn, 
London,  W. 

71,  Lavender  Hill,  S.E. 
12,  New  Union  St.,  E.C. 

35,  Russell  Square,  W.C. 


Telegraphic  Address  and 
Telephone  No. 


Kinmacolor,  Lon- 
don. 

T.  N.,  City,  3976. 


5,  Gray’s  Inn  Passage, 
W.C. 

Wollishofen,  Zurich 


Rotatoria,  London . 
Wall,  1109. 
Central,  4124. 

Sentido,  London. 
Central,  8722. 
Dryplate,  Zurich. 
T.  N.,  484. 

Urban.  See  Natural  Colour  Kinematograph  Company,  Ltd. 
Utocolor.  Soci6t6  La  Garenne  - Colombes, 

Anonyme  Uto-  Paris, 
color. 

Watkins  Meter  Co.  Imperial  Mills,  Hereford  Watkins, Hereford. 
Wratten  & Wain-  Croydon,  Surrey  Wratten,  Croydon, 

wright  Croydon,  572. 


INDEX 


A 

Abney,  Sir  W.,  on  rendering 
plate  sensitive  to  red  rays,  30 
Absorption  of  light,  coefficient 
of,  15 

Acid  colours,  191 
„ definition  of,  190 
Anastigmat  lenses,  96 
Aniline  dyes,  191 

„ „ firms  which  sup- 

ply, 192 

Apertures  between  leaves  form- 
ing circles  on  negative,  23 
Aplanat  lenses,  97 
Appearance  of  white  on  Auto- 
chrome positive,  how  pro- 
duced, 90,  91 

Appendix  A.  Theories  of  colour 
vision,  259 

Table  1.  Exposure  times  for 
colour  plates,  263 
„ 2.  Exposure  times  for 
sunsets,  265 

„ 3.  Colour  Synthesis, 

266 

„ 4.  Relative  brightness 

of  parts  of  spec- 
trum, 266 

„ 5.  Slowest  exposures 

necessary  to  secure 
sharpness,  267 
,,  6.  Factor  numbers  for 

developments,  267 
„ 7.  Developers  for  Au- 

tochromes, 270 
„ 8.  Lumiere’s  formula, 

1908, 271 


Table  9.  Lumiere’s  gradu- 
ated formula, 1910, 
272 

,,  9a.  Prof.  Namias’  for- 
mula for  develop- 
ment of  plates, 

274 

,,  10.  Other  developers, 

275 

,,  11.  Intensification  for- 
mulae, 276 

„ 12.  Reductionformulae, 
277 

,,  13.  Instructions  for  de- 
veloping Omnico- 
lore plates,  278 

,,  14.  Instructions  for  de- 
veloping Dufay 
plates,  280 

„ 15.  Instructions  for  de- 
veloping Paget 
plates,  282 

,,  16.  Elimination  of 
green  spots,  283 

,,  17.  Recent  devices  for 
protecting  colour 
slide  in  the  lan- 
tern, 284 

,,  18.  Sensitising  and  re- 
sensitising colour 
plates,  284 

,,  19.  Colour-screen  fil- 
ters and  mono- 
chromatic light, 
285 

,,  20.  Hitchins’  deve- 
loper for  Auto- 
chromes, 286 


296 


INDEX 


Table  21.  Metric  equivalent 
tables,  287 

„ 22.  English  and  foreign 

sizes  of  plates,  290 

,,  23.  Comparative  speed 

of  plates,  290 

„ 24.  Wave  lengths  of 

visible  spectrum, 
291 

,,  25.  Thermometric 

scales,  292 

,,  26.  List  of  firms  men- 

tioned in  this 
work,  293 

Art  in  colour  photography,  241 
Autochrome  plates,  description 
of,  84 

„ ,,  instructions 

for  deve- 
lop  ing, 
Table  7, 
Appendix, 
270  - 275 ; 
also  286 

,,  screen,  remarkable 

resemblance  to  oil-globule 
colour  screen  in  the  eyes  of 
certain  birds  and  reptiles,  45 


B 

Background  being  a dirty 
colour,  cause  of,  127 
Backgrounds,  choice  of,  256 
Base,  definition  of,  190 
Basic  colours,  191 
Becquerel  rays,  8 
Binding  the  plates  (separate 
methods),  122 

Black  conditions  of  McDo- 
nough, 89 

,,  true  meaning  of  sensa- 
tion, 54,  55 

,,  spots  in  positive,  124 


Bleach-out  process,  theory  of, 
180 

,,  ,,  details  of, 

186 

,,  „ theory  of 

Grothus, 
182 

,,  „ law,  Smith’s, 

183 

Blind  spot,  description  of,  55 
Blisters  in  film,  127 
Boll,  discovery  of  visual  purple, 
53 

Butler’s  three-plate  camera,  149 
,,  details  for  working, 
155 


0 

Camera,  selection  of,  for  colour 
work,  96 

Carbon  colour  process,  174 
Carrara  method  of  printing 
autochromes,  132 
Choice  of  lens,  97 
,,  of  plate,  93 
,,  of  subject,  99 
Chromatic  circle,  243 
Cinemacolor.  See  Kinemacolor, 
203 

Cinematograph.  See  Kinema- 
tograph,  203 

Clearing  the  image,  109,  112 
Clouds,  colours  of,  15 
Colloid,  definition  of,  190 
Collotype  colour  process,  161 
Colour  blindness,  47-54 
,,  ,,  total,  47 

Colour  carbon  process,  170 
„ filter  for  colour  plates, 
101,  137 

„ filters  for  monochroma- 
tic light,  137, 
138,  285 


INDEX  297 


Colour  filters  for  Aut.oclirome 
and  other  colour 
plates,  145 

„ ,,  effect  of  thick- 

ness, 103,  147 
,,  ,,  testing  of,  148 

, , cause  of  sensation  of , 1 1 , 
12 

„ formation,  theory  of, 
67 

,,  how  produced,  12,  13 
, , photography,  history  of, 

Chap.  II.,  29 

,,  plates,  comparison  be- 
tween, 73 
,,  vision,  61 
„ theories  of,  52,  and  Ap- 
pendix, 259 

„ permanent  and  fugi- 

tive, 182 

„ pictures,  stereoscopic 

effect,  138 

,,  plates,  defects  in,  123 
„ positive,  processes  con- 
cerned in,  108 

Coloured  lights,  effect  on  dyes, 
184 

Colours,  acid  and  basic,  191 
,,  additive  and  subtrac- 

tive, 48,  68 

,,  complementary,  69, 

243 

,,  of  screen  plate,  why 

insufficient,  46 
„ manufacturers  of,  192 

,,  primary,  secondary, 

and  tertiary,  241 
,,  pure,  where  found,  50 

,,  surface,  18 

Combined  and  separate  plates 
compared,  78,  79 

Condition,  black,  first  and 
second,  89,  90 

Cones  convey  colour  sense,  34- 
47 

Corpuscular  theory  of  light,  2 


Creme  de  Menthe,  cause  of 
colour  in,  16 

Curves  of  sensitivity  of  plates 
and  of  the  eye,  56-59 


D 

Dark  room  lamp,  106 
Defects  in  colour  plates,  123 
Development, 

instructions  for, 

„ Autochrome  plate, 
Tables  7,  8,  9, 10, 
Appendix,  263- 
292 

„ Omnicolore  plate, 
Table  14,  Appen- 
dix, 278 

„ Dufay  plate,  Table 

14,  Appendix,  280 
,,  Paget  plate,  Table 

15,  Appendix,  282 
Development,  first,  108 

,,  second,  111 

,,  of  screen  plate, 

general  in- 
structions, 81, 
82 

,,  for  uncertain  ex- 

posures, Table  9,  Appendix, 
273 

Dichroic  colours,  17 
„ fog,  123 
Dioptichrome  plate,  76 
Dufay’s  plate  described,  76 
Dyes,  how  affected  by  coloured 
light,  184 

,,  method  of  increasing 
sensitivity  of,  189 
,,  nature  of,  189 
,,  manufacturers,  list  of, 
192 

,,  tabulated  list  of,  191 
, , list  of  firms  which  supply 

them,  192 


298 


INDEX 


E 

Edridge-Green  on  colour,  50 

„ on  colour  blind- 
ness, 51,  52 

Ether,  3 

„ waves,  4 

Evolution  of  colour  photo- 
graphy, Chap.  II.,  29-33 

Exposure  of  plate,  rules  for,  105, 
also  Appen- 
dix, 263- 
264 

„ „ uncertain,  de- 

veloper for,  Table  9,  Appen- 
dix, 272 

Eye,  analogy  of,  34 

,,  compared  with  a camera, 
Chap.  III.,  34 

,,  .description  of  natural 
colour  filter  in,  44 


F 

Face  of  portrait  appears  thin 
and  eaten  away,  128 
Faraday,  Michael,  7 
Fatigue  of  retina,  48,  49 
Film  broken,  what  to  do,  127 
,,  scratches  in,  127 
Filters  for  Autochrome  and 
other  colour 
plates,  145 

,,  „ monochromatic 

light,  138 

Focal  length  of  eye,  40 
Forster,  Prof.,  on  the  produc- 
tion of  white  in  a colour 
plate,  90,  91 

Fovea  ( see  Macula),  37,  44, 
45 

Fresnel,  2,  10 
Frilling  of  film,  127 
Fugitive  colours,  182 


G 

Gaumont  cinematography  in 
colours,  212 

Goethe’s  Farbenlehre,  29 
Goodall,  T.  E.,  on  colour  effects, 
13,  60 

Grant  on  protecting  plate  in 
slide,  96 

,,  on  resensitising  colour 
plates,  136 

Green  (Edridge  - Green)  on 
colour  sense,  50-52 
Green  spots,  removal  of,  283 
Grothus’  bleach-out  law,  182 

H 

Half-tone  processes  in  colour, 
158 

Hardening  the  film,  112 
Hauron,  Ducos  du,  31 
Helmholtz,  31 

„ colour  theory,  259 
Hering’s  theory  of  colour,  Ap- 
pendix, 261 

Hitchins’  developer,  286 
Hood,  use  of,  104 
Hubl,  Baron  Yon,  on  theory  of 
intensification  of  light,  86 
Huyghens  on  wave  theory  of 
light,  2 

Hypo  fixing  bath,  cause  of  re- 
ducing image,  127 

I 

Image,  final  improvement  of, 
121 

„ reversal  of,  111 
Indoor  portraiture,  132 
Insertion  of  plate  in  slide,  100 
Intensification  by  mercury,  116, 
117,  and  Ap- 
pendix, 276 


INDEX 


299 


Intensification  by  pyro  and  sil- 
ver, 116,  and 
Appendix,  276 
,,  of  image,  113 

,,  theory  of,  113 

Interference  of  light  explained, 
63 

Irregular  plates,  character  of,  73 
Isocyanine,  effect  of,  31 

J 

Johnson,  Lindsay,  Dr. — 
Explanation  of  yellow  colour 
of  macula,  44 

Existence  of  yellow  filter  in 
the  eye,  44 

Similarity  between  auto- 
chrome coloured  starch 
layer,  and  layer  of  coloured 
oil  globules  in  birds  and 
reptiles,  45,  46 
Explanation  of  use  of  visual 
purple,  53 

Explanation  of  how  white  is 
primed  in  an  autochrome 
picture,  90,  91 

Joly’s  ruled-line  screen  process, 
71,  72 

Jougla’s  Omnicolore  plate,  76 
K 

Kerr  phenomenon,  8 
Kinemacolor  projection  lan- 
tern, 209 

„ camera,  208 

„ projection  of 

pictures,  209 
,,  principle  of,  204 

Konig,  Dr.,  on  panchromatic 
plates,  32 

„ Pinatype  process, 
166 

Kromskop,  Ives’,  142 


L 

Lambert’s  law,  15 
Lamp,  dark  room,  for  colour 
plates,  105 

Lantern  projection  of  colour 
positives,  134,  284 
Lens,  choice  of,  97 

„ best  focal  length  to  use,  98 
,,  human,  rapidity  of,  38 
Light,  corpuscular  theory  of,  2 
,,  electromagnetic  theory 
of,  7 

„ filters,  preparation  of, 
137 

,,  interference  of,  63-67 
,,  nature  of,  1 
,,  sources  of,  1 
,,  white,  nature  of,  9 
Lippmann,  Prof.,  photography 
by  interference  colours,  30, 
43,  63-67 
Lucas,  H.,  46 

Lumiere’s  Autochrome  plate,  84 
„ screen  anticipated  by 
birds  and  reptiles,  46 


M 

Macula  lutea,  description  of, 
37,  44,  45 

„ ,,  why  yellow,  44 

Manufacturers  of  dyes  and 
stains,  list  of,  192 

Mariotte’s  blind  spot,  54,  55 

Massiot’s  heat  - protecting 
lantern,  284 

Maxwell,  Clerk,  theory  of 
light,  7 

McDonough’s  two  black  con- 
ditions, 89,  90 

Monochromatic  light,  filters 
for,  138 

Mercury  intensifier,  116,  and 
Appendix,  276 


300  INDEX 


Miethe,  Prof.,  on  sethyl  red,  31 
„ on  choice  of  lens, 
98 

Mother-of-pearl,  interference 
colours  of,  66 

Muscles  of  the  eye,  what  is 
their  purpose  in  animals,  38 


N 

Newton,  Sir  Isaac,  2 

,,  and  Lucas  on  the  use 
of  grey,  46 
Newton’s  rings,  66 
Nicol  prism,  61 

0 

Obernetter  on  dyeing  the  film, 
30 

Omnicolore  plates  (Jougla’s) 
described,  76 

Over-exposure,  remedy  for,  123 
P 

Paget  plate  (separate)  de- 
scribed, 79, 80 
„ ,,  (combined)  de- 

scribed, 84 
Paper,  Utocolor,  192 
Parallax,  explanation  of,  75 
Permanent  colours,  182 
Persistence  of  vision,  204 
Photomicrography  in  colours — 
Low  power,  215 
High  power,  225 
Pigments,  colours  of,  how 
caused,  18 
Pinacyanol,  31,  57 
Pinatype  process,  166 
Plate,  choice  of,  93,  94 

,,  combined  and  separate, 
78,  79 


Plate,  insertion  of,  in  slide,  99 
Plates,  resensitising,  135,  284 
Portraiture  indoors,  131 
Positive,  disappearance  of  co- 
lour in,  129 
,,  drying,  118 
,,  dull  and  opaque  ap- 
pearance of,  128 
„ protecting  by  glass, 
119 

,,  red  and  orange  spots 
in,  129 

„ thin  appearance  of, 
128 

Powders,  colour  of,  14,  15 
Printing  by  Utocolor  paper,  195 
Process,  carbon  colour,  170 
„ Pinatype,  166 
,,  Sanger-Shepherd’s  im- 
bibition, 163 

,,  three-colour  half-tone, 
158 

Projection  of  transparencies  in 
colour  on  screen,  134 
Purkinje  phenomenon,  59,  60 
„ „ proof  of, 

60,  61 

R 

Raydex  process,  171 
Rayleigh,  Lord,  on  colour  of 
sky,  27 

Red  tone  in  positive,  126 
Reduction,  methods  of,  117, 
and  Appendix,  277 
Reflection,  theory  of,  21 
Regular  plates,  character  of,  73 
Resensitising  plates,  135,  284 
Retina,  description  of,  34 
Reversal  of  image,  how  ob- 
tained, 111 

Rod  vision  and  cone  vision, 
34-47 

Rods  act  as  dampers,  39 
„ double  function  of,  39 


INDEX 


301 


S 


T 


Salt,  definition  of  a,  190 
Sanger-Shepherd,  32 

,,  ,,  imbibition  pro- 

cess of  colour  photography, 
163 

Screen  plates,  comparison  be- 
tween, 73 

Second  development  in  colour 
photography,  110 
Sensitising  plates,  284 
Shadows,  22,  250 

„ coloured,  24,  25 
,,  production  of,  245 

,,  why  black,  24 

Silver  intensifies,  116,  and 
Appendix 

Single-plate  processes,  theory 
of,  86,  87 

Sky,  colour  of,  26,  27 
Smith,  Dr.  J.  H.,  inventor  of 
„ Utopaper,  33 

,,  bleach-out  law,  183 
Smith  and  Urban,  33 
Smith’s  Kinemacolor  projec- 
tion, 203 

Soap-hubbles,  colour  of,  due  to 
interference,  66 
Speeds  of  plates  compared, 
85 


Spots  (black)  in  positive,  124 
„ (green)  „ 125 

„ (white)  ,,  125 

„ (red)  „ 125 

„ (orange)  „ 129 

Stains  (brown)  ,,  124 

,,  (yellow)  ,,  123 

„ manufacturers  of,  182 
Stereoscopic  effect  produced  by 


colour,  135 
Subject,  choice  of,  98 
Szczepanik,  32,  33 


Table  showing  characteristic 
features  of  colour  plates, 
74 

Tapetum  Lucidum,  43 
Tar  colours  due  to  interference, 
66 

Testing  colour  filters,  148 
Thames  plates  described,  77 
Thin  positive,  123 
Three-colour  half-tone  process, 
158 

„ „ photography, 

theory  of,  141 

„ ,,  negatives, making 

of,  155 

,,  „ printing,  67,  68 

Three-plate  camera  (Butler’s), 
154 

Translucency,  20 
Transparency,  only  relative,  20 
Two-plate  colour  photography, 
152 

Tyndall  on  clouds,  27 


U 

Underexposure,  remedy  for, 
123 

Urban  - Smith’s  Kinemacolor 
method,  203 
,,  principle  of  Kine- 
macolor explained,  204 
Utocolor,  fixing  the  print,  199 
,,  lantern  slides,  202 

, , methods  of  improving 

the  print,  198 
„ paper,  192 

,,  paper  printing,  196 

,,  rapid  printing  colour 

paper,  193 

,,  stripping  paper,  200 


302 


INDEX 


v 

Varnishing  the  plate,  119 

Veiled  fog,  126 

Violet-blue  tone  in  image,  cause 
of,  126 

Virida  paper  for  dark-room 
lamp,  107 

Vision,  persistence  of,  205 

Visual  purple,  use  of,  51,  52 

Vogel,  rendering  plates  sensi- 
tive to  orange  rays,  30 


W 

Wave  theory  of  light,  5 
White,  how  produced  in  an 
Autochrome,  90,  91 
„ light,  nature  of,  9 


Wiener,  theory  of  bleaching,  30 
Wrattens  K1  filter,  103 


Y 

Yellow,  a true  sensation,  20,  44 
„ not  a primary  colour,  19 

„ spot,  reason  for  its 

colour,  44 

„ stains  in  plate,  123,  124 
Young,  Thomas,  2,  31 
Young-Helmholtz’  theory  of 
colour,  Appendix  A,  259 


Z 

Zeeman  effect,  8 

Zenker,  stationary  waves,  30, 31 


PRINTED  BY  WILLIAM  CLOWES  AND  SONS,  LIMITED, 
LONDON  AND  BECCLES,  ENGLAND. 


GETTY  RESEARCH  INSTITUTE 

| ' i ! 1 ^ 

3 3125  01142  8501 


